Coal Mine Respirable Dust Standard Noncompliance Determinations

DEPARTMENT OF LABOR

Mine Safety and Health Administration

Coal Mine Respirable Dust Standard Noncompliance Determinations

Correction and Republication

Note: For the convenience of the user, notice document 97-33937

is being reprinted in its entirety because of numerous errors in the

document originally appearing at 62 FR 68395-68420, December 31,

1997. Those wishing to see a listing of corrections, please call

Patricia Silvey, Mine Safety and Health Administration, 703-235-

1910.

AGENCY: Mine Safety and Health Administration, Labor.

ACTION: Notice; final policy.

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SUMMARY: This notice announces the Mine Safety and Health

Administration's (MSHA) final policy concerning the use of single,

full-shift respirable dust measurements to determine noncompliance and

issue citations, based on samples collected by MSHA, when the

applicable respirable dust standard is exceeded. This notice should be

read in conjunction with the notice published elsewhere in today's

Federal Register jointly by the Department of Labor and the Department

of Health and Human Services.

EFFECTIVE DATE: This policy is effective March 2, 1998.

FOR FURTHER INFORMATION CONTACT: Ronald Schell, Chief, Division of

Health, Coal Mine Safety and Health; MSHA; 703-235-1358.

SUPPLEMENTARY INFORMATION:

I. About This Notice

This notice provides information about MSHA's new enforcement

policy for the use of single, full-shift respirable dust measurements

obtained by inspectors to determine noncompliance with the respirable

dust standard (applicable standard) under the MSHA coal mine respirable

dust program. A question and answer format has been used to explain the

background for the enforcement policy, the reasons for the policy

change, and the specific elements of the new policy. In addition,

several appendices are attached to and incorporated with this final

notice which address technical issues concerning the new enforcement

policy.

II. Background Information

A. How Has MSHA Sampled Coal Mines for Noncompliance in the Past?

Prior to October 1975, noncompliance determinations were based on

the average of full-shift measurements collected from individual

occupations on multiple shifts. MSHA interprets a full shift for

underground coal mines to mean the entire shift worked or 8 hours in

duration or whichever time period is less (30 CFR 70.201(b)). The need

to reduce the Agency's administrative burden attributable to inspector

sampling prompted MSHA to revise its underground health inspection

procedures and redirect the Agency's enforcement resources away from

sampling and toward assessing the effectiveness of mine operators'

respirable dust control programs.

Since October 1975, MSHA has determined noncompliance with the

applicable standard based on the average of measurements obtained for

different occupations during the same shift of a mechanized mining unit

(MMU), or on the average of measurements obtained for the same

occupation on successive days. The term MMU is defined in 30 CFR

70.2(h) to mean a unit of mining equipment, including hand loading

equipment, used for the production of material. MSHA inspectors

routinely sample multiple occupations to determine compliance with the

applicable standard, assess the effectiveness of mine operators' dust

control programs, determine whether excessive levels of quartz dust are

present, and verify the designation of the ``high risk occupation''

(now referred to as the ``designated occupation'' or ``D.O.''--the

occupation on a working section exposed to the highest respirable dust

concentration) to be sampled by mine operators.

Under the sampling procedures in place between 1975 and 1991, MSHA

inspectors would collect full-shift measurements from the working

environment of the ``D.O.'' and four other occupations, if available,

on the first day of sampling each MMU. The mine operator was cited if

the average of all measurements obtained during the same shift exceeded

the applicable standard by at least 0.1 milligram of respirable dust

per cubic meter of air (mg/m3). If one or more measurements

exceeded the applicable standard but the average did not, the Agency's

practice was to continue sampling for up to four additional production

shifts or days. If the inspector continued sampling after the first day

because a previous measurement exceeded the applicable standard,

noncompliance determinations were based on either the average of all

measurements taken or on the average of measurements taken on any one

occupation. Thus, if the average of measurements taken over more than

one day on all occupations was less than or equal to the applicable

standard, but the average of measurements taken on any one occupation

exceeded the value set by MSHA (based on the cumulative concentration

for two or more measurements exceeding 10.4 mg/m3, which is

equivalent to a 5-measurement average exceeding 2.0 mg/m3),

the operator was cited for exceeding the applicable standard.

In some instances, MSHA inspectors sampled for a maximum of five

production shifts or days before making a noncompliance determination.

However, most citations issued prior to 1991 were based on the average

of multiple measurements on different occupations collected during a

single shift. To illustrate, MSHA conducted a computer simulation using

data from 3,600 MMU inspections conducted between October 1989 and June

1991. This simulation showed that a total of 293 MMUs would have met

the criteria to be found in noncompliance with the applicable standard

based solely on the average of multiple measurements. Two hundred

forty-two of those noncompliance determinations, or 83 percent, met the

citation criteria based on sampling results from the first day of MSHA

sampling, rather than from multi-day sampling. Only 51 MMUs, or 17

percent, were citable based on the average of measurements collected

over multiple shifts or days. These statistics clearly show that the

citation criteria were met based not only on the average of

measurements taken during several shifts, but also on the average of

[[Page 5688]]

multiple measurements obtained during the same shift.

B. Why Did MSHA Establish the Coal Mine Respirable Dust Task Group and

Initiate the Spot Inspection Program?

In 1991 concerns were raised about the adequacy of MSHA's program

to control respirable coal mine dust in underground coal mines. In

response to these issues, MSHA established the Coal Mine Respirable

Dust Task Group (Task Group) to comprehensively evaluate the

effectiveness of the Agency's respirable dust program.

The Task Group was directed to consider all aspects of the current

program, including the role of the individual miner in the sampling

program; the feasibility of MSHA conducting all sampling; and the

development of new and improved monitoring technology, including

technology to continuously monitor the mine environment. Among the

issues addressed by the Task Group was the actual dust concentration to

which miners are exposed. As a result, the Agency initiated a special

respirable dust ``spot inspection program'' (SIP), designed to provide

the Agency with more accurate information on the dust levels to which

miners were exposed, through sampling, in the underground coal mine

environment.

C. How Was Sampling Accomplished During the SIP?

Because of the large number of mines and MMUs involved and the need

to obtain data within a short time frame, sampling during the SIP was

limited to a single shift or day, a departure from MSHA's normal

sampling procedures. As a result, the Agency determined that if the

average of multiple occupation measurements taken on an MMU during any

one-day inspection did not exceed the applicable standard, the

inspector would review the result of each sample individually. If any

individual measurement exceeded the applicable standard by an amount

specified by MSHA, a citation would be issued for noncompliance,

requiring the mine operator to take immediate corrective action to

lower the average dust concentration.

The sampling practice under the SIP was similar to the practice of

the Metal/Nonmetal Health Division of MSHA, and the Occupational Safety

and Health Administration (OSHA), which use a single, full-shift

measurement for noncompliance determinations, and provides for a margin

of error to account for uncertainty in the measurement process

(sampling and analytical error). This resulted in the issuance of

citations using a single, full-shift measurement only when there was a

high level of confidence that the applicable standard was actually

exceeded.

Thus, during the SIP inspections, MSHA inspectors cited violations

of the current 2.0 mg/m\3\ standard if either the average of five

measurements taken on a single shift was greater than or equal to 2.1

mg/m\3\, or any single, full-shift measurement was greater than or

equal to 2.5 mg/m\3\. Similar adjustments were made when the 2.0 mg/

m\3\ standard was reduced due to the presence of quartz (crystalline

silica) dust in the mine environment.

D. What Did the SIP Show About MSHA's Sampling Policy?

MSHA's review of the SIP inspections showed that 28 percent of 718

MMUs sampled exceeded the applicable standard and would have been

citable based on a single, full-shift measurement, but only 12 percent

would have been citable using the average of all measurements for the

MMU.

Based on the data from the SIP inspections, the Task Group

concluded that the Agency practice of determining noncompliance based

solely on the average of multiple measurements did not always reveal

situations in which miners were overexposed. For example, if the

measurements obtained for five different occupations within the same

MMU were 4.1, 1.0, 1.0, 2.5, and 1.4 mg/m\3\, the average concentration

would be 2.0 mg/m\3\ and no enforcement action would be taken, even

though the dust measurements for two of these occupations significantly

exceeded the applicable standard. While such individual measurements

were not cited prior to the SIP, they would clearly demonstrate that

some miners were overexposed. MSHA policy prior to the SIP however,

required the inspector to return to the mine on the next production day

and resume sampling, rather than issue a citation at the time the

overexposures were discovered.

E. Why Did MSHA Decide To Permanently Adopt the SIP Procedures?

The SIP inspections revealed instances of overexposure that were

masked by the averaging of results across different occupations. This

showed that miners would not be adequately protected if noncompliance

determinations were based solely on the average of multiple

measurements. The process of averaging dilutes a high measurement made

at one location with lower measurements made elsewhere. Similarly,

averaging a number of full-shift measurements can obscure cases of

overexposure.

Additionally, the Task Group recognized that the initial full-shift

samples collected by an inspector are likely to show higher dust

concentrations than succeeding samples collected on subsequent shifts

during the same inspection. MSHA's data showed that the average

concentration of all samples taken on the same occupation on the first

day of an inspection was almost twice as high as the average

concentration of those taken on the second day. MSHA recognized that

sampling on successive days after an inspector first appears could

result in measurements that are not representative of dust conditions

to which miners are typically exposed. Unrepresentative measurements

would arise if mine operators anticipated the continuation of inspector

sampling and made adjustments in dust control parameters or production

rates to reduce dust levels during the subsequent monitoring. None of

this is specifically prohibited by MSHA regulations. As a result of

these findings, which indicated that miners were at risk of being

overexposed, MSHA decided to permanently adopt use of the single, full-

shift measurement inspection policy initiated during the SIP. These

procedures were used by MSHA until the issuance of the decision by the

Federal Mine Safety and Health Review Commission in the case of

Keystone Coal v. Sec. of Labor, 16 FMSHRC 6 (Jan. 4, 1994). Since that

decision, MSHA has reverted to its previous practice of basing

noncompliance determinations on the average of multiple, full-shift

measurements. (Please see the notice of joint finding by the Secretary

of Labor and the Secretary of Health and Human Services (HHS) published

elsewhere in today's Federal Register for an explanation of this

decision.)

III. Why MSHA Is Revising Its Enforcement Policy

A. What Has Changed To Warrant Revising the Existing Enforcement

Policy?

During the public hearings held on the proposed joint finding that

a single, full-shift sample is an accurate measurement, during the

public meetings held on this enforcement policy notice, and in other

comments submitted to the Agency, several commenters questioned why the

current program should be altered. The commenters asserted that MSHA's

practice of issuing citations based on the average of multiple

measurements has

[[Page 5689]]

been in effect since the 1970s, that technology and equipment

associated with sampling remain essentially the same, and that

substantial progress had been made in lowering respirable dust levels

at U.S. coal mines.

As stated in the final notice of joint finding published elsewhere

in today's Federal Register, significant improvements have in fact been

made in the dust sampling process. Although MSHA agrees that progress

has been made in reducing average dust concentrations, the SIP

inspections clearly showed instances of excessive dust concentrations

that would have been masked by the procedure of averaging measurements.

Specifically, of the 718 SIP MMUs with valid single, full-shift

measurements, 203 MMUs had at least one single, full-shift measurement

that was citable, while only 88 MMUs met or exceeded the citation

threshold based on the average of multiple measurements. This clearly

shows that under the procedure of averaging measurements miners would

be at risk of being overexposed and MSHA would be unable to require

operators to take corrective actions to protect them.

MSHA believes that a single, full-shift measurement is more likely

to detect excessive dust concentrations and thus protect miners than a

measurement average across multiple occupations on a single shift or

across multiple shifts for a single occupation. MSHA's computer

simulation which analyzed data from over 3600 MMU inspections conducted

between October 1989 and June 1991, showed that 814 MMUs had citable

overexposures based on individual samples, but only 298 of these

overexposures were citable on the average of measurements made within

the MMU. Subsequent to the SIP, between January 1992 and December 1993,

MSHA continued making noncompliance determinations on a single, full-

shift measurement, and 74 percent or 488 of the 658 MMUs cited by

inspectors as having overexposures were found to be out of compliance

based on a single, full-shift measurement, requiring mine operators to

take appropriate corrective action. This experience clearly

demonstrates that citing on a single, full-shift measurement, as

opposed to citing on the average of measurements taken over multiple

shifts, impacts miners directly, because it requires mine operators to

take more prompt corrective action once an overexposure has been

identified. This reduces the risk to miners of continued exposure to

dust concentrations above the applicable standard on subsequent shifts.

Furthermore, both NIOSH, in its recently issued criteria document,

and the Secretary of Labor's Advisory Committee on the Elimination of

Pneumoconiosis Among Coal Mine Workers recommended the use of single,

full-shift measurements for determining compliance. According to the

Committee report, issued in October 1996, the MSHA practice of not

issuing citations based on single, full-shift samples ``is not

protective of miner health, moreover, it is inconsistent with the

stated intent of the Coal Act and the Mine Act, which require that

exposure be at or below the exposure limit for each shift.''

B. Why Will MSHA No Longer Rely On Averaged Measurements of Dust

Concentrations To Determine Noncompliance?

MSHA's current enforcement strategy does not provide the optimal

level of possible health protection. Basing noncompliance

determinations on the average of different occupational measurements

dilutes a measurement of high dust exposure with a lower measurement

made at a different occupational location. Likewise, averaging

measurements obtained for the same occupation over different shifts

does not ensure that the concentration of respirable dust is maintained

at or below the applicable standard during each shift. Section

202(b)(2) of the Mine Act clearly requires that dust concentrations be

maintained at or below the applicable standard ``* * * during each

shift to which each miner in the active workings'' is exposed.

Some commenters proposed that MSHA continue to average at least

five separate measurements prior to making a noncompliance

determination. They stated that abandoning this practice would reduce

the accuracy of noncompliance determinations. Specifically, these

commenters maintain that the average of dust measurements obtained at

the same occupational location on different shifts more accurately

represents dust exposure to a miner than a single, full-shift

measurement. These commenters favored the retention of existing MSHA

policy on the grounds that not averaging measurement results would

reduce accuracy to unacceptable levels. Other commenters agreed with

MSHA that the averaging of multiple samples dilutes measurements of

dust concentration and masks specific instances of overexposure. Some

of these commenters stated that averaging distorts not only the

estimate of dust concentration applicable to individual shifts, but

also biases the estimate of exposure levels over a longer term.

According to these commenters, this is because dust control measures

and work practices affecting dust concentrations are frequently

modified in response to the presence of an MSHA inspector over more

than a single shift. These commenters argued that the presence of the

MSHA inspector causes the mine operator to be more attentive to dust

control than normal.

Section 202(b) of the Mine Act requires each mine operator to

``continuously maintain the average concentration of respirable dust in

the mine atmosphere during each shift to which each miner is exposed''

at or below the applicable standard. The greater the variation in

mining conditions from shift to shift, the less likely it is that a

multi-shift average will reflect the average dust concentration on any

individual shift. For example, during one shift, production may be high

and dust concentrations may also be correspondingly high. However, the

next shift may experience lower production levels because of equipment

breakdowns or because of unusual mining conditions. In addition, when a

mine operator knows that the MSHA inspector is present, more attention

may be given to ensuring that dust control measures operate

effectively, and this may also affect the concentrations of respirable

coal mine dust found on that shift. Because of such factors, multi-

shift averaging does not improve the accuracy of a noncompliance

determination for the sampled shift. Therefore, MSHA is discontinuing

its policy of relying on averaged dust concentrations. A more technical

discussion of how averaging measurements affects accuracy is given in

Appendix A.

C. Why Has MSHA Decided To Base Noncompliance Determinations Solely on

a Single, Full-Shift Measurement?

One commenter suggested that the new enforcement strategy proposed

in MSHA's February 1994 notice, involving noncompliance determinations

based on either a single sample or on the average of multiple samples,

placed operators in ``double jeopardy'' of being cited--that is, it

provided for two separate evaluations of whether the applicable

standard has been exceeded. This commenter pointed out that this

enforcement strategy would reduce the confidence level at which a

noncompliance determination could be made.

Under the MSHA policy proposed in the February 1994 notice,

measurements made by an MSHA inspector for

[[Page 5690]]

different occupational locations would have been averaged together, not

in order to estimate a hypothetical average concentration, but rather

to ascertain whether dust concentration was excessive at any of the

sampled locations. If the average of measurements across sampling

locations exceeded the applicable standard, then at least one of the

sampling locations would almost certainly have been out of compliance

on the sampled shift. Therefore, the commenter was correct in asserting

that noncompliance at each sampling location would have been evaluated

twice: once using the single measurement specific to that location;

and, if that test did not result in a citation, once again using the

average of all available measurements.

MSHA had determined that this strategy was necessary to provide the

level of health protection to miners required by the Mine Act, and

included this strategy in the proposed policy notice to protect against

cases of evident noncompliance that would otherwise go uncited. For

example, if five occupational measurements of 2.08, 2.28, 2.31, 2.25,

and 2.17 mg/m3 were obtained for an MMU on a 2.0 mg/

m3 standard, no enforcement action would be taken if

noncompliance is determined solely based on a single, full-shift

measurement because no individual measurement meets or exceeds the

Citation Threshold Value (CTV), defined in section IV.B. of this

notice. On the other hand, averaging the measurements results in an

average concentration of 2.22 mg/m3, indicating, with high

confidence, that the applicable standard was exceeded.

Although MSHA originally proposed using a combination of both

strategies for determining noncompliance, various bodies of data show

that such hypothetical occurrences are extremely improbable in

practice. For example, MSHA's computer simulation discussed earlier in

this notice showed that, between October 1, 1989, and June 30, 1991,

298 MMUs would have been found in noncompliance with the applicable

standard based on averaging multiple measurements. All 298 MMUs would

also have been found in noncompliance using the single, full-shift

measurement citation criteria. According to the data from the SIP, only

one noncompliance determination would have been missed if all averaging

had been discontinued. Similarly, analysis of more recent inspector

sampling data for 1995 indicates that miners' health will not be

compromised by discontinuing all measurement averaging. In fact, only

one additional case of noncompliance would have been identified using

averaging in addition to citing on a single, full-shift measurement.

Therefore, MSHA will not continue to use this combination of

strategies.

As explained in the final notice of joint finding published

elsewhere in today's Federal Register, MSHA's improved sampling and

analytical method performs in accordance with the NIOSH Accuracy

Criterion whenever a single, full-shift measurement is at or above 0.36

mg/m3. The Agency believes that, in accordance with section

202(f) of the Mine Act, this enables MSHA to base noncompliance

determinations on a single, full-shift measurement whenever that

measurement is at or above 0.36 mg/m3.

IV. The New Enforcement Policy

A. What Is MSHA's New Enforcement Policy?

MSHA will continue its current dust sampling program as it relates

to where and how many samples an inspector collects during a sampling

shift. Specifically, MSHA will continue to collect multiple

occupational samples for each MMU. The criterion for making

noncompliance determinations has been revised and, under the new

enforcement policy, MSHA will use a control filter capsule to adjust

the resulting weight gain obtained on each exposed filter capsule.

Noncompliance determinations will be based solely on the results of

individual, full-shift samples, and MSHA will issue a citation whenever

noncompliance is demonstrated at a high confidence level. The Agency

will no longer rely on multi-locational or multi-shift averaging of

measurements to determine noncompliance.

The process by which a violation of the applicable standard will be

abated by a mine operator will also remain unchanged. MSHA will

consider a violation to be abated when samples collected in accordance

with 30 CFR 70.201(d) demonstrate that the average dust concentration

in the working environment of the cited occupation is at or below the

applicable standard.

When a measurement exceeds the applicable standard but is less than

the CTV, noncompliance is not demonstrated at a sufficiently high

confidence level to warrant a citation. However, MSHA will consider

whether to target the MMU or environment for additional dust sampling.

See Appendix B for further discussion of why MSHA believes that such

measurements indicate probable overexposure.

B. When Will MSHA Issue a Citation for a Violation of the Applicable

Standard?

MSHA will issue a citation for noncompliance when a single, full-

shift measurement demonstrates, at a high level of confidence, that the

applicable standard has been exceeded. Although MSHA will continue to

collect multiple occupational samples for each MMU, the Agency will

generally issue only one citation for exceeding the applicable standard

on a single shift on any one MMU. However, additional citations may be

issued when excessive dust concentrations are detected for occupations

exposed to different dust generating sources.

To ensure that citations are issued only when there is a high level

of confidence that the applicable standard has been exceeded, MSHA has

developed the Citation Threshold Values (CTV) below. Each CTV listed is

calculated so that citations are issued only when the single, full-

shift measurement demonstrates noncompliance with at least 95 percent

confidence. Citing in accordance with the CTV table does not constitute

a raising of the applicable standard. Instead, it reflects the need for

MSHA to ensure a sufficiently high level of confidence in its

noncompliance determinations. Mine operators are still required to

implement appropriate controls that will maintain the average

concentration of respirable dust at or below the applicable standard on

all shifts.

Citation Threshold Values (CTV) for Citing Violations Based on Single,

Full-Shift Measurements

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Applicable standard (mg/m3) CTV (mg/m3)

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2.0................................................. 2.33

1.9................................................. 2.22

1.8................................................. 2.11

1.7................................................. 2.00

1.6................................................. 1.90

1.5................................................. 1.79

1.4................................................. 1.68

1.3................................................. 1.58

1.2................................................. 1.47

1.1................................................. 1.36

1.0................................................. 1.26

0.9................................................. 1.15

0.8................................................. 1.05

0.7................................................. 0.94

0.6................................................. 0.84

0.5................................................. 0.74

0.4................................................. 0.64

0.3................................................. 0.53

0.2................................................. 0.43

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C. How Will the CTV Table Be Applied?

Each single, full-shift measurement used to determine noncompliance

will

[[Page 5691]]

be the MRE-equivalent dust concentration as calculated and recorded

under MSHA's dust data processing system. Every valid measurement will

be compared with the CTV corresponding to the applicable standard in

effect. If any measurement meets or exceeds that value, a citation will

be issued. However, no more than one citation will be issued based on

single, full-shift measurements from the same MMU, unless separate

citations are warranted for occupations exposed to different dust

generating sources. Therefore, when single, full-shift measurements

from two or more occupations show dust concentrations in violation of

the applicable standard, as illustrated in the examples below, the

inspector will determine the dust generation sources and require the

operator to sample the environment of the occupation most affected by

these sources which is consistent with current practice. In most cases,

this will be the working environment of the ``D.O.'' However, if

noncompliance is indicated based on measurements from two or more

occupations on the same MMU which are exposed to the same dust

generating sources, and which do not involve the ``D.O.,'' the

occupation with the highest dust concentration will be identified in

the citation as the affected working environment. In any case, when an

inspector issues a citation for violation of the applicable standard

under the new policy, the citation narrative will identify the specific

environment or occupation to be sampled by the operator, as well as any

other occupation(s) that exceeded the CTV.

Several commenters requested that the application of the CTV table

be clarified. The following examples illustrate how inspectors will

apply the CTV table and make noncompliance determinations. Suppose that

a measurement of 2.41 mg/m3 is obtained for the ``D.O.'',

and measurements of 2.34, 1.54, and 1.26 mg/m3, are obtained

for three other occupations exposed to the same dust generating sources

as the ``D.O.'' during a single shift on an MMU required to comply with

an applicable standard of 2.0 mg/m3. Because at least one of

the measurements exceeds the 2.33-mg/m3 CTV (the citation

value when the applicable standard is 2.0 mg/m3), a citation

will be issued for exceeding the applicable standard on the shift

sampled. Even though two individual measurements (2.41 and 2.34 mg/

m3) exceeded the CTV, one of which is on the ``D.O.,'' only

one citation will be issued, specifying the ``D.O.'' as the affected

working environment because all occupations were exposed to the same

dust generating sources.

Suppose now that in the previous example the 2.34-mg/m3

measurement was obtained for a roof bolter, and the MMU was ventilated

using a double-split ventilation system. This means that the roof

bolter, working on a separate split of air from that of the continuous

miner, is exposed to a different dust generating source than the

``D.O.'' and, therefore, may not be adequately protected by dust

controls implemented for the ``D.O.'' Consequently, two citations would

be issued.

As another example, consider an MMU with measurements of 2.14,

1.92, 1.82, 1.25, and 1.12 mg/m3. Although none of these

measurements meet the CTV, there is reason to believe that the MMU is

out of compliance, since one of the measurements exceeds the applicable

standard. However, because there is a small chance that the measurement

exceeded the applicable standard because of measurement error, a

citation would not be issued. As discussed elsewhere in this notice,

additional samples would be necessary to verify the adequacy of the

control measures under current operating conditions. Therefore, MSHA

would select this MMU for additional sampling. As discussed in Appendix

B, even if the first measurement were 1.90 mg/m3 instead of

2.14 mg/m3, because of measurement error this would not

demonstrate that the mine atmosphere sampled was in compliance. To

confirm that control measures are adequate, MSHA would need to take

additional samples.

D. What Is the Potential for a Citation To Be Issued Due To Measurement

Error?

Some commenters expressed concern that noncompliance determinations

based on single, full-shift measurements would result in an

unacceptable number of erroneous citations due to measurement error.

These commenters expected that MSHA's new enforcement policy would

result in numerous erroneous citations.

Based on the analysis in Appendix C, MSHA has concluded that,

because of the large ``margin of error'' separating each CTV from the

corresponding applicable standard, use of the CTV table provides ample

protection against erroneous citations. For exceptionally well-

controlled environments (e.g., Case 2 of Appendix C), the probability

that any given citation is erroneous will be substantially less than 5

percent. This probability is even smaller in environments which are not

well controlled (e.g., Case 3 of Appendix C). Therefore, any citation

issued in accordance with the CTV table will be much more likely the

result of excessive dust concentration rather than measurement error.

E. What Will Happen When the Evidence Is Insufficient To Warrant a

Citation?

If the appropriate CTV is not met or exceeded, MSHA will not issue

a citation. As discussed earlier, this does not mean that the sampled

environment is necessarily in compliance. Although in certain cases

there may be insufficient evidence to demonstrate noncompliance, the

measurement may nonetheless indicate a possible overexposure. MSHA

intends to focus on cases of measurements above the applicable standard

but below the CTV, with special emphasis being directed to working

environments required to comply with applicable standards below 2.0 mg/

m3.

If follow-up measurements do not warrant a citation but suggest

that the dust control measures in use may be inadequate, MSHA may

initiate a thorough review of the dust control parameters stipulated in

the mine operator's approved ventilation or respirable dust control

plan to determine whether the parameters should be upgraded.

V. Consequences of the Use of the CTVs in Conjunction With the

Joint MSHA/NIOSH Finding

A. What is the Impact of MSHA's New Enforcement Strategy As Applied

Under the MSHA/NIOSH Joint Finding?

The Agency believes that the application of the CTVs in conjunction

with the MSHA/NIOSH joint notice of finding published elsewhere in

today's Federal Register to single, full-shift samples collected by

MSHA inspectors provides for more efficient detection of noncompliance

by identifying and requiring abatement of individual instances of

overexposure which meet the CTVs. While this issue is more

appropriately addressed in the MSHA/NIOSH joint notice, the rationale

for this conclusion bears repeating here.

The Mine Act is clear in its intent that no miner should be exposed

to respirable coal mine dust in excess of the applicable standard on

any shift. The effect of the joint finding and the new enforcement

strategy set forth here creates incentives for mine operators to

control dust exposure on a continuing basis to minimize the chance of

being found in noncompliance during any MSHA sampling inspection. To

prevent the possibility of any inspector single,

[[Page 5692]]

full-shift measurement exceeding the CTV and resulting in a violation,

mine operators will be more likely to keep dust concentrations at or

below the applicable standard, thereby providing better protection to

miners from overexposures. This becomes evident upon closer examination

of the inspector sampling data from the period when noncompliance

determinations were based on single, full-shift measurements.

MSHA reviewed inspector MMU sampling results for FY 1992, the first

full year during which noncompliance determinations were based on

single, full-shift measurements, and FY 1993, the last year that the

Agency issued citations based on single, full-shift measurements. This

review showed a decline in the number of ``D.O.'' and nondesignated

occupation samples exceeding 2.0 mg/m3, from 16 percent and

10 percent in FY 1992 to 13 percent and 7 percent, respectively, in FY

1993, suggesting that operators were better able to maintain dust

concentrations below the applicable standard. MSHA also conducted a

computer simulation using these data which showed that one of every

four MMU sampling days in FY 1992 would have been found in

noncompliance based on a single, full-shift measurement, compared to

one in five MMU sampling days in FY 1993.

Under the previous enforcement strategy, which utilized averaging,

inspectors cited violations of the applicable standard on the average

of multiple measurements taken on a single shift or on different shifts

or days. Consequently, dust concentrations could be excessive for some

occupations or work locations, but corrective action would not be

required so long as the average of the measurements did not exceed the

applicable standard. For example, averaging occupational measurements

of 3.2, 2.4, 1.5, 1.3 and 1.0 mg/m3 results in an average

concentration of 1.8 mg/m3 for the sampled MMU where the

applicable standard is 2.0 mg/m3. Despite the fact that two

of the measurements demonstrate noncompliance with a high degree of

confidence, corrective action would not have been required because the

average concentration was below the applicable standard.

As described in this notice and in conjunction with the MSHA/NIOSH

joint notice, under the new enforcement policy, whenever an individual

measurement indicates noncompliance (with a high level of confidence),

the mine operator will be required to take corrective action to lower

the concentration of respirable dust to comply with the applicable

standard.

Some commenters expressed concern that MSHA would fail to cite some

instances of noncompliance because of the high level of confidence

required for a citation. MSHA believes that the new enforcement

strategy as applied in conjunction with the finding of the MSHA/NIOSH

joint notice will reduce the chances of failing to cite cases of

noncompliance as compared to the previous policy of measurement

averaging, while at the same time ensuring that noncompliance is cited

only when there is a high degree of confidence that the applicable

standard has been exceeded. According to the inspector sampling

inspections conducted in 1995, only 132 MMUs were found to be in

violation of the applicable standard and cited under the previous

enforcement policy of measurement averaging, compared to 545 MMUs that

would have been citable under the new enforcement policy in conjunction

with the joint notice of finding using single, full-shift measurements.

This clearly demonstrates that the new enforcement policy, in

conjunction with the joint notice, will not compromise miners' health

but would, instead, have identified 413 additional instances of

overexposure that would have gone unaddressed under the previous policy

of measurement averaging.

Some commenters proposed that miners would be even more protected

if noncompliance was cited whenever any single, full-shift measurement

exceeded the applicable standard by any amount. That is, it was

recommended that MSHA not make any allowance for potential measurement

errors. MSHA has considered this recommendation but has not adopted it

in the final policy because it could result in citations being issued

where compliance with the applicable standard is more likely than not.

If the mine environment is sufficiently well controlled, it is more

likely that a particular measurement exceeds the applicable standard,

but not the CTV, due to measurement error rather than due to excessive

dust concentration. Furthermore, the rationale used by these commenters

to justify their proposed citation criterion breaks down when, as in

the case of multiple samples taken during a given shift in the same

MMU, more than one measurement is made for a single noncompliance

determination. Appendix D addresses technical details relating to this

issue.

Some commenters stated that MSHA's new citation criteria

implemented in conjunction with the joint notice will not improve

respirable dust levels in the environment, but will simply result in

MSHA issuing more citations to mine operators. In these commenters

view, this will foster a continuation of the adversarial relationship

that developed between mine operators and MSHA over allegations of

widespread tampering with respirable dust samples.

MSHA firmly believes that basing noncompliance determinations on a

single, full-shift measurement will improve working conditions for

miners because it will cause mine operators to either implement and

maintain more effective dust controls to minimize the chance of being

found in noncompliance by an MSHA inspector, or take corrective action

sooner to lower dust concentrations that are shown, with high

confidence, to be in excess of the applicable standard. The effect of

this new enforcement policy in conjunction with the MSHA/NIOSH joint

notice will be remedial in nature because it will address instances of

overexposure that are not addressed under the current policy of

measurement averaging. For example, between January 1992 and December

1993, MSHA continued the practice established under the SIP of making

noncompliance determinations based on single, full-shift measurements

which demonstrated, with high confidence, that the applicable standard

was exceeded, and on the average of multiple measurements. During this

period, MSHA inspectors issued a total of 658 citations at MMUs. The

majority of these citations (488) were issued based on the result of a

single, full-shift measurement. Under the existing enforcement policy,

such individual instances of noncompliance would not be cited and

corrected, but instead would be factored into an average that could be

at or below the applicable standard, resulting in no violation and no

corrective action taken by the mine operator.

Some commenters also contended that the joint notice of finding,

and this notice of policy, are solely for the administrative

convenience of MSHA's mine inspectors. The commenters stated that

allowing inspectors to make noncompliance determinations on the basis

of a single, full-shift measurement will eliminate the need for

inspectors to sample on successive days, as is sometimes required under

existing policy.

MSHA recognizes that there are administrative advantages related to

the adoption of this new enforcement policy and the joint notice of

finding. By eliminating the need to sample on subsequent days, the

Agency will be able to utilize its resources more efficiently. That is,

inspectors will not

[[Page 5693]]

be required to return to a mine to conduct additional dust sampling,

but the Agency will be able to redirect its resources to other safety

and health concerns. This result is consistent with the Mine Act's

objective of protecting miner safety and health. While administrative

convenience may be a side benefit of this new enforcement policy in

conjunction with the MSHA/NIOSH joint notice, the primary reason for

implementing it is to achieve the intent of Congress that no miner

shall be exposed to dust concentrations above the applicable standard

on any shift.

B. What is the Impact of the New Policy on Ventilation Plans?

A number of commenters expressed concern that issuing citations on

the result of a single, full-shift measurement will cause MSHA to

require carefully developed ventilation plans to be modified needlessly

as part of the abatement process. These commenters view such frequent

revisions as costly, disruptive and unnecessary. They contend that such

revisions, if required, would be made on the basis of incomplete or

invalid information, and that they would not necessarily decrease a

miner's dust exposure. Some commenters believed that some inspectors

would mandate specific changes without realistically evaluating their

effectiveness, while other inspectors would not allow operators to make

their own adjustments to the plans, or provide an opportunity for them

to evaluate the changes in a rational manner.

When a citation is issued based on a single measurement, this can

indicate that the control measures in use may no longer be adequate to

maintain the environment within the applicable standard. MSHA will

consequently review the adequacy of the ventilation plan under the

current operating conditions, and will consider the results of operator

bimonthly sampling as well as operator compliance with the approved

ventilation plan parameters. Under this approach MSHA would require

plan revisions only after an examination of all factors has

demonstrated that changes are necessary to protect miner health. This

enforcement strategy should minimize unnecessary changes to plans that

have been determined to provide adequate controls.

MSHA believes that the primary focus of the federal dust program is

to minimize miners' overexposures to respirable dust through the

application of appropriate environmental controls, which are stipulated

in the operator's approved mine ventilation plan. After these controls

are evaluated and shown to be effective under typical mining

conditions, if properly maintained, they should provide reasonable

assurance that no miner will be overexposed. Therefore, one of the

objectives of MSHA's dust sampling is to verify that the controls

stipulated in ventilation plans continue to adequately control dust

concentrations under existing operating conditions. In conjunction with

these sampling and other inspections an inspector checks and measures

the dust control parameters early in the shift to determine whether the

approved ventilation plan is being followed. A mine operator's failure

to follow the parameters stipulated in the plan will result in the

issuance of a citation, which requires immediate corrective action to

abate the violation. The type of corrective actions taken to abate plan

violations can vary from unplugging clogged water sprays to increasing

the amount of ventilating air delivered to the MMU. However, mere

correction of these deficiencies to ensure that the ``status quo'' of

the plan is being maintained may not always be effective in controlling

miners' exposure to respirable dust. The required plan parameters may

no longer be effective in maintaining compliance, and may need to be

upgraded. The determination of how the plan should be revised is

complicated by the fact that, generally, most approved plans do not

incorporate all the control measures that were in place when MSHA

sampled. Consequently, most plan revisions have simply incorporated

into the plan only those dust controls that were in use when MSHA

sampled, rather than requiring significant upgrading of the plan. As an

example, an MSHA inspector might require an increase in the water

pressure stipulated in the plan from 75 pounds per square inch (psi) to

125 psi to reflect the 125 psi that the MSHA inspector actually

measured. If, instead, the operator was required to significantly

increase the quantity of air being delivered to the MMU, this would be

considered a major upgrade. MSHA recognizes that a determination of

noncompliance should not automatically necessitate the revision of a

plan. Instead, it should result in a thorough review of the plan's

continued adequacy.

When an operator of an underground mine is cited for excessive

dust, 30 CFR 70.201(d) requires the operator to ``take corrective

action to lower the concentration of respirable dust to within the

permissible concentration.'' When the citation is based on MSHA

samples, the inspector may request that the operator describe what type

of corrective action will be taken. The inspector then determines if

the corrective action is appropriate. If it is not appropriate in the

specific situation, the inspector may either suggest or require other

corrective action or control measures. Operators are provided with the

opportunity to make adjustments to their dust controls and to evaluate

their effectiveness in a rational manner during the time for abatement

set by the inspector, which is based on the complexity of the problem,

availability of controls, and the types of changes the operator intends

to make. This abatement time may be extended by the inspector based on

the operator's performance in reducing the dust concentration in the

affected area of the mine. Typically, the operator then demonstrates,

through sampling, that the underlying condition or conditions causing

the violation have been corrected. Failure to take corrective action

prior to sampling that shows continuing noncompliance may lead to the

issuance of a withdrawal order. However, this occurs infrequently.

C. Will the New Enforcement Policy Increase Citations on Individual

Shifts, Even if the So-Called ``Average Concentration Over the Longer

Term'' Meets the Standard?

Some commenters claimed that even when the average dust

concentration is well below the applicable standard, normal variability

from shift to shift results in a substantial fraction of shifts for

which the dust standard is exceeded. According to these commenters, a

determination of noncompliance is warranted only if the average dust

concentration to which a miner is exposed exceeds the standard over a

period of time greater than a single shift, such as a bimonthly

sampling period, a year, or a miner's working lifetime. Therefore, they

consider it ``unfair'' to cite operators for exceeding the applicable

standard on individual shifts, so long as the average over the longer

term meets the applicable standard. For example, based on historical

sampling data provided by one commenter, the commenter concluded that,

``* * * there is at least a 1 in 6 or 17% probability that any single

sample can show potential overexposure when one does not exist.'' These

commenters contend that use of the CTV to determine noncompliance,

based on one sample collected on a single shift, will substantially

increase the frequency of ``unfair'' citations, compared to existing

MSHA policy.

MSHA believes that such comments reflect a misunderstanding of both

the

[[Page 5694]]

requirements of the Mine Act and MSHA's longstanding policy with

respect to single, full-shift noncompliance determinations. It should

be recognized that MSHA has been basing noncompliance determinations on

the average of multiple occupation measurements obtained on the same

shift since 1975. In addition, some of the commenters confused the

average dust concentration over the course of an individual shift with

the average dust concentration over some longer term. The joint notice

of finding issued by the Secretaries of Labor and HHS addresses this

issue. Since the Mine Act requires that dust concentration be kept

continuously at or below the applicable standard on every shift, it is

appropriate to cite noncompliance when any single, full-shift

measurement at a particular location demonstrates, with high

confidence, that the applicable standard has been exceeded on an

individual shift.

Section 201(b) of the Mine Act mandates that MSHA ensure ``to the

greatest extent possible, that the working conditions in each

underground coal mine are sufficiently free of respirable dust

concentrations * * * to permit each miner the opportunity to work

underground during the entire period of his adult life without

incurring any disability from pneumoconiosis or any other occupation-

related disease during or at the end of such a period.'' Since neither

past nor future exposure levels can be assumed for any miner, MSHA's

enforcement strategy must be to limit the exposure on every shift as

intended by the Mine Act.

D. Will There Be Any Changes in Operator Bimonthly Sampling?

Several commenters were unclear about the impact of the joint MSHA/

NIOSH finding and this policy on operator sampling for compliance and

for abatement of violations. One commenter suggested that 30 CFR

70.207(a) be revised to allow the operator to submit one single, full-

shift sample, instead of five samples every bimonthly period as

currently required. Another commenter suggested that MSHA assume

responsibility for dust sampling from the mine operators.

MSHA has previously noted that the change in its enforcement policy

announced through this final notice affects only how it will determine

noncompliance based on measurements obtained by MSHA inspectors. There

will be no change in how MSHA evaluates operator-collected respirable

dust samples for compliance. Under the regulations currently in effect,

the Agency will continue to average operator samples taken on multiple

shifts or days to make noncompliance determinations. MSHA is committed

to revising procedures with respect to operator-collected respirable

dust samples through the rulemaking process for consistency with this

final finding.

Several commenters expressed concerns about the credibility of the

operator sampling program because of alleged operator tampering with

respirable dust samples and alleged operator manipulation of mine

conditions during dust sampling periods. As a result, these commenters

felt that mine operators should no longer have responsibility for

sampling because their sampling results are unreliable. Another

commenter expressed support for the Agency to compel coal mine

operators to comply with existing dust standards. Another commenter

voiced concern that a mine operator could be wrongly cited due to the

loss or mishandling of a single, full-shift sample by MSHA, and claimed

that such occurrences had happened in the past. Some commenters believe

that if noncompliance can be determined based on a single, full-shift

sample, an operator should be allowed to abate a citation with a

single, full-shift sample, particularly if the operator has recently

demonstrated compliance through bimonthly samples. Another commenter

questioned the impact of the proposed program on the operator's

program, specifically, whether MSHA would require each of the abatement

samples to meet the single, full-shift sample citation threshold

values, in addition to meeting the dust standard based on the average

of five abatement samples.

Issues concerning operator sampling are not germane to this

enforcement policy notice, which concerns only the use of samples

collected by MSHA inspectors. The changes set forth in this final

notice only address how MSHA will determine noncompliance when sampling

is conducted by federal mine inspectors. There is no change in how MSHA

evaluates either operator-collected bimonthly samples or samples taken

to abate a dust citation. MSHA is committed to revising any procedures

with respect to the operator program through the rulemaking process for

consistency with this final finding.

Concerning the credibility of the operator sampling program, MSHA

recognizes that there have been instances of abuse under the current

operator sampling program. The Task Group found that the majority of

operators do not engage in such conduct. MSHA will continue to monitor

the operator sampling program, increase the frequency of inspector

sampling, and target problem mines for additional inspections, as

appropriate.

MSHA processes over 80,000 samples annually and it is not

unrealistic to expect some samples to be either lost in the mail or

accidentally misplaced. MSHA's experience of processing more than 7

million dust samples since 1970 indicates that this occurs

infrequently. In the event a sample is lost, the mine operator is

afforded ample opportunity to submit a replacement sample. If a

citation is issued due to the operator's failure to submit the required

number of samples, the affected operator can present evidence that the

required number of samples had been submitted and request that MSHA

vacate the citation.

E. How Can MSHA Base a Noncompliance Determination on a Single, Full-

Shift Sample, When Five Samples Are Required in Operator Bimonthly

Sampling?

Once a finding has been made that a single, full-shift measurement

will accurately represent atmospheric conditions to which a miner is

exposed during such shift, MSHA is bound by the terms of the Mine Act

to make noncompliance determinations based on single, full-shift

measurements. No regulatory action is required to implement this change

in MSHA's dust sampling program. On the other hand, the present

regulatory scheme for operator sampling was developed based on

noncompliance determinations being made by averaging the results of

multiple samples over five successive shifts or days. In order for MSHA

to incorporate the single, full-shift sample concept into the operator

sampling program, the Agency must revise the operator sampling

regulations through notice and comment rulemaking.

F. Do the New Citation Criteria Have any Impact on Permissible Exposure

Limits?

Some commenters contended that a policy of citing in accordance

with the CTV table, rather than citing whenever a measurement exceeds

the applicable standard, effectively increases the allowable dust

concentration limit. Other commenters stated that the enforcement of

the applicable standard as a limit on each shift, rather than as a

limit on the average concentration over some longer time period,

effectively reduces the standard.

Citing in accordance with the stated CTV neither increases nor

decreases the dust standard. Operators are required to maintain

compliance with the

[[Page 5695]]

applicable standard at all times. MSHA's citing of noncompliance only

when there is high confidence that the applicable standard has been

exceeded does not increase the permissible concentration limit. Again,

mine operators must maintain compliance with the applicable standard.

MSHA requires that dust controls maintain dust concentrations at or

below the applicable standard on all shifts, not merely at or below the

CTV. It is also MSHA's intent under this new enforcement policy that if

a measurement exceeds the applicable standard by an amount insufficient

to warrant citation--that is, the level does not meet or exceed the

CTV--MSHA will target that mine or area for additional sampling to

ensure that dust controls are adequate.

Those commenters who stated that applying the applicable standard

to each shift will effectively reduce the respirable dust standard

overlooked the fact that, since 1975, MSHA has taken enforcement action

based on average of measurements obtained for different occupations

during a single shift. This new enforcement policy does not change

MSHA's interpretation of section 202(b) of the Mine Act that dust

concentrations be maintained at or below the applicable standard on

each shift. The new enforcement policy merely reflects a change in the

technical criteria used to cite violations of the applicable dust

standard.

Appendix A--The Effects of Averaging Dust Concentration Measurements

MSHA's measurement objective in collecting a dust sample is to

determine the average dust concentration at the sampling location on

the shift sampled. As discussed in the joint notice of finding

published elsewhere in today's Federal Register, a single, full-shift

measurement can accurately represent the average full-shift dust

concentration being measured. Nevertheless, because of sampling and

analytical errors inherent in even the most accurate measurement

process, the true value of the average dust concentration on the

sampled shift can never be known with complete certainty. However

accurate the representation, a measurement can provide only an estimate

of the true dust concentration. Some commenters contended that MSHA

should not rely on single samples for making noncompliance

determinations, because an average of results from multiple samples

would estimate the true dust concentration more accurately than any

single measurement.

Contrary to the views expressed by these commenters, averaging a

number of measurements does not necessarily improve the accuracy of an

estimation procedure. Consider, for example, an archer aiming at

targets mounted at random and possibly overlapping positions on a long

partition. Each arrow might be aimed at a different target. Suppose

that an observer, on the opposite side of the partition from the

archer, cannot see the targets but must estimate the position of each

bull's eye by locating protruding arrowheads.

Each protruding arrowhead provides a measurement of where some

bull's eye is located. If two arrowheads are found on opposite ends of

the partition, averaging the positions of these two arrowheads would

not be a good way of determining where any real target is located. To

estimate the location of an actual target, it would generally be

preferable to use the position of a single arrow. The average would

represent nothing more than a ``phantom'' target somewhere near the

center, where the archer probably did not aim on either shot and where

no target may even exist.

The archery example can be extended to illustrate conditions under

which averaging dust concentration measurements does or does not

improve accuracy. If each arrowhead is taken to represent a full-shift

dust sample, then the true average dust concentration at the sampling

location on a given shift can be identified with the location of the

bull's eye at which the corresponding arrow was aimed. The accuracy of

a measurement refers to how closely the measurement can be expected to

come to the quantity being measured. Statistically, accuracy is the

combination of two distinct concepts: precision, which pertains to the

consistency or variability of replicated measurements of exactly the

same quantity; and bias, which pertains to the average amount by which

these replicated measurements deviate from the quantity being measured.

Bias and precision are equally important components of measurement

accuracy.

To illustrate, arrows aimed at the same target might consistently

hit a sector on the lower right side of the bull's eye. The protruding

arrowheads would provide more or less precise measurements of where the

bull's eye was located, depending on how tightly they were clustered;

but they would all be biased to the lower right. On the other hand, the

arrows might be distributed randomly around the center of the bull's

eye, and hence unbiased, but spread far out all over the target. The

protruding arrowheads would then provide unbiased but relatively

imprecise measurements.

More complicated situations can easily be envisioned. Arrows aimed

at a second target would provide biased measurements relative to the

first target. Alternatively, if the archer always aims at the same

target, the first shot in a given session might tend to hit near the

center, with successive shots tending to fall off further and further

to the lower right as the archer's arm tires; or shots might

progressively improve, as the archer adjusts aim in response to prior

results.

Averaging reduces the effects of random errors in the archer's aim,

thereby increasing precision in the estimation procedure. If the archer

always aims at the same target and is equally adept on every shot

(i.e., if the arrowheads are all randomly and identically distributed

around a fixed point), then averaging improves the estimate's precision

without introducing any bias. Averaging in such cases provides a more

accurate method of estimating the bull's eye location than reliance on

any single arrowhead. If, however, the archer intentionally or

unintentionally switches targets, or if the archer's aim progressively

deteriorates, then averaging can introduce or increase bias in the

estimate. If the gain in precision outweighs this increase in bias,

then averaging several independent measurements may still improve

accuracy. However, averaging can also introduce a bias large enough to

offset or even surpass the improvement in precision. In such cases, the

average position of several arrowheads can be expected to locate the

bull's eye less accurately than the position of a single arrowhead.

I. Multi-Locational Averaging

Some commenters opposed MSHA's use of a single, full-shift

measurement for enforcement purposes, claiming that determinations

based on such measurements would be less accurate than those made under

MSHA's existing enforcement policy of averaging multiple measurements

taken on an MMU. There are two distinctly different types of multi-

locational measurement averages that could theoretically be compiled on

a given shift: (1) the average might combine measurements taken for

different occupational locations and (2) the average might combine

measurements all taken for the same occupational location. For MMUs,

the averages used in MSHA's sampling program usually involve

measurements taken for different occupational locations on the same

shift. These are averages of the first type. MSHA's sampling program

has never utilized

[[Page 5696]]

averages of the second type. Therefore, those commenters who claimed

that reliance on a single, full-shift measurement would reduce the

accuracy of noncompliance determinations, as compared to MSHA's

existing enforcement policy, are implicitly claiming that accuracy is

increased by averaging across different occupational locations.

Averaging measurements obtained from different occupational

locations on an MMU is like averaging together the positions of arrows

aimed at different targets. The average of such measurements is an

artificial, mathematical construct that does not correspond to the dust

concentration for any actual occupational location. Therefore, this

type of averaging introduces a bias proportional to the degree of

variability in actual dust concentration at the various locations

averaged.

The gain in precision that results from averaging measurements

taken at different locations outweighs this bias only if variability

from location to location is smaller than variability in measurement

error. However, commenters opposed to MSHA's use of single, full-shift

measurements for enforcement purposes argued that this is not generally

the case and even submitted data and statistical analyses in support of

this position. Commenters in favor of noncompliance determinations

based on a single, full-shift measurement agreed that variability in

dust concentration is extensive for different occupational locations

and argued that MSHA's existing policy of measurement averaging is not

sufficiently protective of miners working at the dustiest locations.

Since an average of the first type combines measurement from the

dustiest location with measurements from less dusty locations, it must

always fall below the best available estimate of dust concentration at

the dustiest location. In effect, averaging across different

occupational locations dilutes the dust concentration observed for the

most highly exposed occupations or dustiest work positions. Therefore,

such averaging results in a systematic bias against detecting excessive

dust concentrations for those miners at greatest risk of overexposure.

A somewhat better case can be made for the second type of multi-

locational averaging, which combines measurements obtained on the same

shift from a single occupational location. As some commenters pointed

out, however, there is ample evidence that spatial variability in dust

concentration, even within relatively small areas, is frequently much

larger than variability due to measurement error. Therefore, the same

kind of bias introduced by averaging across occupational locations

would also arise, but on a lesser scale, if the average measurement

within a relatively small radius were used to represent dust

concentration at every point in the atmosphere to which a miner is

exposed. A miner is potentially exposed to the atmospheric conditions

at any valid sampling location. Consistent with the Mine Act and

implementing regulations, MSHA's enforcement strategy is to limit

atmospheric dust concentration wherever miners normally work or travel.

Therefore, the more spatial variability in dust concentration there is

within the work environment, the less appropriate it is to use

measurement averaging to enforce the applicable standard by averaging

measurements obtained at different sampling locations.

Some of the comments implied that instead of measuring average dust

concentration at a specific sampling location, MSHA's objective should

be to estimate the average dust concentration throughout a miner's

``breathing zone'' or other area near a miner. If estimating average

dust concentration throughout some zone were really the objective of

MSHA's enforcement strategy, then averaging measurements made at random

points within the zone would improve precision of the estimate without

introducing a bias. This type of averaging, however, has never been

employed in either the MSHA or operator dust sampling programs. MSHA's

current policy of averaging measurements obtained from different zones

does not address spatial variability in the area immediately

surrounding a sampler unit. Therefore, even if averaging measurements

from within a zone were somehow beneficial, this would not demonstrate

that MSHA's existing enforcement policy is more reliable than the new

policy of basing noncompliance on a single, full-shift measurement.

Furthermore, if MSHA's objective were really to estimate average

dust concentration throughout some specified zone on a given shift,

then it would be necessary to obtain far more than five simultaneous

measurements within the zone. This is not only because of potentially

large local differences in dust concentration. In order to use such

measurements for enforcement purposes, variability in dust

concentration within the sampled area would have to be estimated along

with the average dust concentration itself. As some commenters

correctly pointed out, doing this in a statistically valid way would

generally require at least twenty to thirty measurements. One of these

commenters also pointed out that such an estimate, based on even this

many measurements in the same zone, could be regarded as accurate only

under certain questionable assumptions about the distribution of dust

concentrations. This commenter calculated that hundreds of measurements

would be required in order to avoid these tenuous assumptions. Clearly,

this shows that the objective of estimating average dust concentration

throughout a zone is not consistent with any viable enforcement

strategy to limit dust concentration on each shift in the highly

heterogeneous and dynamic mining environment. The large number of

measurements required to accurately characterize dust concentration

over even a small area merely demonstrates why it is not feasible to

base enforcement decisions on estimated atmospheric conditions beyond

the sampling location.

MSHA recognizes that a single, full-shift measurement will not

provide an accurate estimate of average dust concentration anywhere

beyond the sampling location. The Mine Act, however, does not require

MSHA to estimate average dust concentration at locations that are not

sampled or to estimate dust concentration averaged over any zone or

region of the mine, and doing so is not part of MSHA's enforcement

program. Instead, MSHA's enforcement strategy is to ensure that a miner

will not be exposed to excessive dust wherever he/she normally works or

travels. This is accomplished by maintaining the average dust

concentration at each valid sampling location at or below the

applicable standard during each shift.

II. Multi-Shift Averaging

Some commenters maintained that in order to reduce the risk of

erroneous noncompliance determinations, MSHA should average

measurements obtained from the same occupation on different shifts.

These commenters contended that the average of measurements from

several shifts represents the average dust concentration to which a

miner is exposed more accurately than a single, full-shift measurement.

Other commenters, who favored noncompliance determinations based on

single, full-shift measurements, claimed that conditions are sometimes

manipulated so as to produce unusually low dust concentrations on some

of the sampled shifts. These commenters suggested that, due to these

[[Page 5697]]

unrepresentative shifts, multi-shift averaging can yield

unrealistically low estimates of the dust concentration to which a

miner is typically exposed. Some of these commenters also argued that

the Mine Act requires the dust concentration to be regulated on each

shift, and that multi-shift averaging is inherently misleading in

detecting excessive dust concentration on an individual shift.

Those advocating multi-shift averaging generally assumed that a

noncompliance determination involves estimating a miner's average dust

exposure over a period longer than an individual shift. This assumption

is flawed because section 202(b) of the Mine Act specifies that each

operator shall continuously maintain the average concentration of

respirable dust in the mine atmosphere during each shift at or below

the applicable standard. Some of those advocating multi-shift

averaging, however, suggested that MSHA should average measurements

obtained on different shifts even if the quantity of interest is dust

concentration on an individual shift. These commenters argued that

averaging smooths out the effects of measurement errors, and that

therefore the average over several shifts would represent dust

concentration on each shift more accurately than the corresponding

individual, full-shift measurement.

The Secretary recognizes that there are circumstances, not

experienced in mining environments, under which averaging across shifts

could improve the accuracy of an estimate for an individual shift. Just

as averaging the positions of arrows aimed at nearly coinciding targets

might better locate the bull's eye than the position of any individual

arrow, the gain in precision obtained by averaging dust concentrations

observed on different shifts could, under analogous circumstances,

outweigh the bias introduced by using the average to estimate dust

concentration for an individual shift. This would be the case, however,

only if variability in dust concentration among shifts were small

compared to variability due to measurement imprecision. It would do no

good to average the location of arrows aimed at different targets

unless the targets were at nearly identical locations.

To the contrary, several commenters pointed out that variability in

dust concentration from shift to shift tends to be much larger than

variability due to measurement error and introduced evidence in support

of this observation. Measurements on different shifts are like arrows

aimed at widely divergent targets. The more that conditions vary, for

any reason, from shift to shift, the more bias is introduced by using a

multi-shift average to represent dust concentration for any individual

shift. Under these circumstances, any improvement in precision to be

gained by simply averaging results is small compared to the bias

introduced by such averaging. Therefore, the Secretary has concluded

that MSHA's existing practice of averaging measurements collected on

different shifts does not improve accuracy in estimating dust

concentration to which a miner is exposed on any individual shift. To

paraphrase one commenter, averaging Monday's exposure measurement with

Tuesday's does not improve the estimate of Monday's average dust

concentration.

Some commenters argued that since the risk of pneumoconiosis

depends on cumulative exposure, MSHA's objective should be to estimate

the dust concentration to which a miner is typically exposed and to

identify cases of excessive dust concentration over a longer term than

a single shift. Other commenters claimed that a multi-shift average

does not provide a good estimate of either typical dust concentrations

or exposures over the longer term. These commenters claimed that

different shifts are not equally representative of the usual

atmospheric conditions to which miners are exposed, implying that the

average of measurements made on different shifts of a multi-day MSHA

inspection tends to systematically underestimate typical dust

concentrations.

The Secretary interprets section 202(b) of the Mine Act as

requiring that dust concentrations be kept at or below the applicable

standard on each and every shift. Nevertheless, the Secretary

recognizes that, under certain conditions, the average of measurements

from multiple shifts can be a better estimate of ``typical''

atmospheric conditions than a single measurement. This applies,

however, only if the sampled shifts comprise a random or representative

selection of shifts from whatever longer term may be under

consideration. As shown below, evidence to the contrary exists,

supporting those commenters who maintained that measurements collected

over several days of a multi-day MSHA inspections do not meet this

requirement. Therefore, the Secretary has concluded that averaging such

measurements is likely to be misleading even for the purpose of

estimating dust concentrations to which miners are typically exposed.

Whether the objective is to measure average dust concentration on

an individual shift or to estimate dust concentration typical of a

longer term, the arguments presented for averaging across shifts all

depend on the assumption that every shift sampled during an MSHA

inspection provides an unbiased representation of dust exposure over

the time period of interest.1 To check this assumption, MSHA

performed a statistical analysis of multi-shift MSHA inspections

carried out prior to the SIP. This analysis, placed into the record in

September 1994, examined the pattern of dust concentrations measured

over the course of these multi-shift inspections and compared results

from the final shift with results from a subsequent single-shift

sampling inspection [1].

---------------------------------------------------------------------------

\1\ Technically, the assumption is that dust concentrations on

all shifts sampled are independently and identically distributed

around the quantity being estimated.

---------------------------------------------------------------------------

The analysis found that dust concentrations measured on different

shifts of the same MSHA inspection were not randomly distributed. The

later samples tended to show significantly lower results than earlier

samples, indicating that dust concentrations on later shifts of a

single inspection may decline in response to the presence of an

inspector. Furthermore, the analysis provided evidence that the

reduction in dust concentration tends to be reversed after the

inspection is terminated. These two results led to the conclusion that

averaging dust concentrations measured on different shifts of a multi-

day MSHA inspection introduces a bias toward unrealistically low dust

concentrations.

One commenter questioned the validity of this analysis, stating

that ``there is absolutely no basis in the * * * report for the

assertion that the trend is reversed after the inspection is

terminated.'' This commenter apparently overlooked Table 3 of the

report. That table shows a statistically significant reversal at those

mine entities included in the analysis that were subsequently inspected

under MSHA's SIP. Dust concentrations measured at these mine entities

had declined significantly between the first and last days of the

multi-shift inspection. It was primarily to address the commenter's

implication that these reductions reflected permanent ``adjustments in

dust control measures'' that the analysis included a comparison with

the subsequent SIP inspection. An increase, representing a reversal of

the previous trend, was observed on the single shift of the subsequent

[[Page 5698]]

inspection, relative to the dust concentration measured on the final

shift of the previous multi-shift inspec tion. This reversal was found

to be ``statistically significant at a confidence level of more than

99.99 percent.''

The same commenter also stated that MSHA ``* * * fails to address

the systematic [selection] bias of the study. MSHA only does multiple

day sampling when the initial results are higher, but not out of

compliance.'' It is true that in order to be selected for revisitation,

a mine entity must have shown relatively high concentrations on the

first shift--though not, in the case of an MMU, so high as to warrant a

citation on first shift. Since no experimental data were available on

mine entities randomly selected to receive multi-shift inspections, the

only cases in which patterns over the course of a multi-shift

inspection could be examined were cases selected for multi-shift

inspection under these criteria.

Although the impact of the selection criteria was not explicitly

addressed, it was recognized that entities selected for multi-day

inspections do not constitute a random selection of mine entities. This

recognition motivated, in part, the report's comparison of the final

shift measurement to the dust concentration measured during a

subsequent single-shift inspection. The magnitude of the average

reversal indicates that most of the reduction observed over the course

of the multi-shift inspection cannot be attributed to the selection

criteria. Furthermore, it was not only mine entities with relatively

low dust concentration measurements that were left out of the study

group. Mine entities with the highest dust concentration measurements

were immediately cited based on the average of measurements taken and

excluded from the group subjected to multi-shift dust inspections.

Therefore, the effect on the analysis of selecting mine entities with

relatively high initial dust concentration measurements was largely

offset by the effect of excluding those entities with even higher

initial measurements. In any event, the magnitude of the average

reduction between first and last shifts of a multi-shift inspection was

significantly greater than what can be explained by selection for

revisitation due to measurement error on the first shift sampled.

The assumption that multiple shifts sampled during a single MSHA

inspection are equally representative is clearly violated if, as some

commenters alleged, operating conditions are deliberately altered after

the first shift in response to the continued presence of an MSHA

inspector and then changed back after the inspector leaves. However, if

samples are collected on successive or otherwise systematically

determined shifts or days, the assumption can also be violated by

changes arising as part of the normal mining cycle. As one commenter

pointed out, multi-shift averaging within a single MSHA inspection

potentially introduces biases typical of ``campaign sampling,'' in

which observations of a dynamic process are clustered together over a

relatively narrow time span. In order to construct an unbiased, multi-

shift average for each phase of mining activity, it would be necessary

to collect samples from several shifts operating under essentially the

same conditions. Alternatively, to construct an unbiased, multi-shift

estimate of dust concentration over a longer term, it would be

necessary to collect samples from randomly selected shifts over a

period great enough to reflect the full range of changing conditions.

Neither requirement is met by multi-shift MSHA inspections because (1)

the mine environment is dynamic and no two shifts are alike and (2)

MSHA inspectors are not there long enough to observe every condition in

their inspection.

Based on the analysis presented by Kogut [1] and also on public

comments received in response to the February 18 and June 6, 1994,

notices, the Secretary has concluded that it should not be assumed that

multiple shifts sampled during a single MSHA inspection are equally

representative of atmospheric conditions to which a miner is typically

exposed. This conclusion undercuts the rationale for multi-shift

averaging within a single MSHA inspection, regardless of whether the

objective is to estimate dust concentration for the individual shifts

sampled as it is for MSHA inspector sampling or for typical shifts over

a longer term as implied by some commenters. Measurements collected by

MSHA on consecutive days or shifts of the same inspection do not

comprise a random or otherwise representative sample from any larger

population of shifts that would properly represent a long-term exposure

or a particular phase of the mining cycle. Therefore, there is no basis

for assuming that multi-shift averaging improves accuracy or reduces

the risk of an erroneous enforcement determination.

Appendix B--Citation Threshold Values (CTV)

I. Interpretation of the CTV Table

Each CTV was calculated to ensure that, if the CTV is met or

exceeded, noncompliance with the applicable standard can be inferred

with at least 95-percent confidence. It is assumed that whatever dust

standard happens to be in effect at the sampling location is binding,

and that a citation is warranted whenever there is sufficient evidence

that an established standard has been exceeded. The CTV table does not

depend on how the applicable standard was established, or on any

measurement uncertainties in the process of setting the applicable

standard.

Some commenters argued that in order to construct a valid table of

CTVs, MSHA would have to take into account the statistical distribution

of dust concentrations over many shifts and locations. One commenter

suggested that stochastic properties of the dust concentrations, which

describe variability over time in probabilistic terms, should also be

taken into account. MSHA, however, intends to use single, full-shift

measurements only in determining noncompliance with the applicable

standard on a particular shift and at the sampling location consistent

with the measurement objective described in the MSHA and NIOSH joint

finding published elsewhere in today's Federal Register. This is

analogous to using a single measurement to identify individual

suitcases that are unacceptable because they weigh more than five

pounds. The efficacy of using a single measurement to identify

unacceptable suitcases depends on the accuracy of the scale and the

skill of the weigher. It does not depend on the statistical

distribution of weights among suitcases or on any stochastic properties

of the suitcase production process. These considerations would be

relevant to estimating average weight for all suitcases produced, but

they have nothing whatsoever to do with determining the weight of an

individual suitcase using a sufficiently accurate scale. Averaging the

weights of several suitcases would be entirely inappropriate and

extremely misleading, since the object is to identify individual

suitcases weighing more than five pounds. Although the measured weight

of an individual suitcase is liable to contain some error (so the

decision might be uncertain for a suitcase weighing five pounds and one

ounce), a suitcase weighing seven or eight pounds could be rejected

with high confidence on the first weighing. Additional weighings (of

the same suitcase) would be required only for those suitcases whose

initial measurement was very close to five pounds.

The CTV table provides criteria for testing a tentative, or

presumptive,

[[Page 5699]]

hypothesis that the true full-shift average dust concentration did not

exceed the applicable standard (S) at each of the individual locations

sampled during a particular shift. For purposes of this test, the mine

atmosphere at each such location is presumed to be in compliance unless

the corresponding full-shift measurement provides sufficient evidence

to the contrary. The ``true full-shift average'' does not refer, in

this context, to an average across different occupations, locations, or

shifts. Instead, it refers entirely to the dust concentration at the

specific location of the sampler unit, averaged over the course of the

particular shift during which the measurement was obtained. The CTV

table is not designed to estimate or test the average dust

concentration across occupational locations, or within any zone or mine

area, or in the air actually inhaled by any particular miner.

Some commenters questioned why more than one sample might be

required, if the first sample collected does not exceed the CTV. One of

these commenters argued that in such case, ``compliance has already

been established at a 95% confidence level based on the first single

shift sample.'' This line of argument confuses confidence in issuing a

citation with confidence of compliance. It also shows a basic

misunderstanding of how the citation criteria relate to the requirement

of continuous compliance under section 202(b) of the Mine Act.

The CTV table ensures that noncompliance is cited only when there

is a 95-percent level of confidence that the applicable standard has

actually been exceeded. If a single measurement does not meet the

criterion for citation, this does not necessarily imply probable

compliance with the dust standard--let alone compliance at a 95-percent

confidence level. For example, a single, full-shift measurement of 2.14

mg/m3 would not, according to the CTV table, indicate

noncompliance with sufficient confidence to warrant a citation if S =

2.0 mg/m3. This does not imply that the mine atmosphere was

in compliance on the shift and at the location sampled. On the

contrary, unless contradictory evidence were available, this

measurement would indicate that the MMU was probably out of compliance.

However, because there is a small chance that the measurement exceeded

the standard only because of measurement error, a citation would not be

issued. Additional measurements would be necessary to verify the

apparent lack of adequate control measures. Similarly, a single, full-

shift measurement of 1.92 mg/m3 would not warrant citation;

but, because of possible measurement error, neither would it warrant

concluding that the mine atmosphere sampled was in compliance. To

confirm that control measures are adequate, it would be necessary to

obtain additional measurements.

Furthermore, even if a single, full-shift measurement were to

demonstrate, at a high confidence level, that the mine atmosphere was

in compliance at the sampling location on a given shift, additional

measurements would be required to demonstrate compliance on each shift.

For example, if S = 2.0 mg/m3, then a valid measurement of

1.65 mg/m3 would demonstrate compliance on the particular

shift and at the particular location sampled. It would not, however,

demonstrate compliance on other shifts or at other locations.

II. Derivation of the CTV Table

Some commenters requested an explanation of the statistical theory

underlying the CTV table. To understand how the CTVs are derived and

justified, it is first necessary to distinguish between variability due

to measurement error and variability due to actual differences in dust

concentration. The variability observed among individual measurements

obtained at different locations (or at different times) combines both:

dust concentration measurements vary partly because of measurement

error and partly because of genuine differences in the dust

concentration being measured. This distinction, between measurement

error and variation in the true dust concentration, can more easily be

explained by first carefully defining some notational abbreviations.

One or more dust samples are collected in the same MMU or other

mine area on a particular shift. Since it is necessary to distinguish

between different samples in the same MMU, let Xi represent

the MRE-equivalent dust concentration measurement obtained from the

ith sample. The quantity being measured is the true, full-

shift average dust concentration at the ith sampling

location and is denoted by i. Because of potential

measurement errors, i can never be known with

complete certainty. A ``sample,'' ``measurement,'' or ``observation''

always refers to an instance of Xi rather than

i.

The overall measurement error associated with an individual

measurement is nothing more than the difference between the measurement

(Xi) and the quantity being measured

(i). Therefore, this error can be represented as

i = Xi-i.

Equivalently, any measurement can be regarded as the true concentration

in the atmosphere sampled, with a measurement error added on:

Xi = i + i.

For two different measurements (X1 and X2), it

follows that X1 may differ from X2 not only

because of the combined effects of 1 and

2, but also because 1 differs

from 2.

The probability distribution of Xi around

i depends only on the probability distribution of

i and should not be confused with the statistical

distribution of i itself, which arises from spatial

and/or temporal variability in dust concentration. This variability

[i.e., among i for different values of I] is not

associated with inadequacies of the measurement system, but real

variation in exposures due to the fact that contaminant generation

rates vary greatly in time and contaminants are heterogeneously

distributed in workplace air.

Since noncompliance determinations are made relative to individual

sampling locations on individual shifts, derivation of the CTV table

requires no assumptions or inferences about the spatial or temporal

pattern of atmospheric dust concentrations--i.e., the statistical

distribution of i. MSHA is not evaluating dust

concentrations averaged across the various sampler locations.

Therefore, the degree and pattern of variability observed among

different measurements obtained during an MSHA inspection are not used

in establishing any CTV. Instead, the CTV for each applicable standard

(S) is based entirely on the distribution of measurement errors

(i) expected for the maximum dust concentration in

compliance with that standard--i.e., a concentration equal to S itself.

If control filters are used to eliminate potential biases, then

each i arises from a combination of four weighing

errors (pre-and post-exposure for both the control and exposed filter

capsule) and a continuous summation of instantaneous measurement errors

accumulated over the course of an eight-hour sample. Since the eight-

hour period can be subdivided into an arbitrarily large number of sub-

intervals, and some fraction of i is associated

with each sub-interval, i can be represented as

comprising the sum of an arbitrarily large number of sub-interval

errors. By the Central Limit Theorem, such a summation tends to be

normally distributed, regardless of the distribution of subinterval

errors. This does not depend on the distribution of

[[Page 5700]]

i, which is generally represented as being

lognormal.

Furthermore, each measurement made by an MSHA inspector is based on

the difference between pre- and post-exposure weights of a dust sample,

as determined in the same laboratory, and adjusted by the weight gain

or loss of the control filter capsule. Any systematic error or bias in

the weighing process attributable to the laboratory is mathematically

canceled out by subtraction. Furthermore, any bias that may be

associated with day-to-day changes in laboratory conditions or

introduced during storage and handling of the filter capsules is also

mathematically canceled out. Elimination of the sources of systematic

errors identified above, together with the fact that the concentration

of respirable dust is defined by section 202(e) of the Mine Act to mean

the average concentration of respirable dust measured by an approved

sampler unit, implies that the measurements are unbiased. This means

that i is equally likely to be positive or negative

and, on average, equal to zero.

Therefore, each i is assumed to be normally

distributed, with a mean value of zero and a degree of variability

represented by its standard deviation

[GRAPHIC] [TIFF OMITTED] TN31DE97.012

Since Xi = i + i, it

follows that for a given value of i, Xi

is normally distributed with expected value equal to

i and standard deviation equal to

i. CVtotal, described in the MSHA and

NIOSH joint finding published elsewhere in today's Federal Register, is

the coefficient of variation in measurements corresponding to a given

value of i. CVtotal relates entirely to

variability due to measurement errors and not at all to variability in

actual dust concentrations.

MSHA's procedure for citing noncompliance based on the CTV table

consists of formally testing a presumption of compliance at every

location sampled. Compliance with the applicable standard at the

ith sampling location is expressed by the relation

i S. Max{i} denotes

the maximum dust concentration, among all of the sampling locations

within an MMU. Therefore, if Max{i} S,

none of the sampler units in the MMU were exposed to excessive dust

concentration. Since the burden of proof is on MSHA to demonstrate

noncompliance, the hypothesis being tested (called the null hypothesis,

or H0,) is that the concentration at every location sampled

is in compliance with the applicable standard. Equivalently, for an MMU

the null hypothesis (H0) is that max{i}

S. In other areas, where only one, full-shift measurement

is made, the null hypothesis is simply that i

S.

The test consists of evaluating the likelihood of measurements

obtained during an MSHA inspection, under the assumption that

H0 is true. Since Xi = i +

i, Xi (or max{Xi} in the case

of an MMU) can exceed S even under that assumption. However, based on

the normal distribution of measurement errors, it is possible to

calculate the probability that a measurement error would be large

enough to fully account for the measurement's exceeding the standard.

The greater the amount by which Xi exceeds S, the less

likely it is that this would be due to measurement error alone. If,

under H0, this probability is less than five percent, then

H0 can be rejected at a 95-percent confidence level and a

citation is warranted. For an MMU, rejecting H0 (and

therefore issuing a citation) is equivalent to determining that

i S for at least one value

of I.

Each CTV listed was calculated to ensure that citations will be

issued at a confidence level of at least 95 percent. As described in

MSHA's February 1994 notice and explained further by Kogut [2], the

tabled CTV corresponding to each S was calculated on the assumption

that, at each sampling location:

[GRAPHIC] [TIFF OMITTED] TN31DE97.013

The MSHA and NIOSH joint finding establishes that for valid

measurements made with an approved sampler unit, CVtotal is

in fact less than CVCTV at all dust concentrations

(i).

The situation in which measurement error is most likely to cause an

erroneous noncompliance determination is the hypothetical case of

i = S for either a single, full-shift measurement

or for all of the measurements made in the same MMU. In that borderline

situation--i.e., the worst case consistent with Ho--the

standard deviation is identical for all measurement errors. Therefore,

the value of s used in constructing the CTV table is the product of S

and CVCTV evaluated for a dust concentration equal to S:

[GRAPHIC] [TIFF OMITTED] TN31DE97.014

Assuming a normal distribution of measurement errors as explained

above, it follows that the probability a single measurement would equal

or exceed the critical value

[GRAPHIC] [TIFF OMITTED] TN31DE97.015

is five percent under Ho when CVtotal =

CVCTV. The tabled CTV corresponding to S is derived by

simply raising the critical value c up to the next exact multiple of

0.01 mg/m3.

For example, at a dust concentration (i) just

meeting the applicable standard of S = 2 mg/m3,

CVCTV is 9.95 percent. Therefore, the calculated value of c

is 2.326 and the CTV is 2.33 mg/m\3\. Any valid single, full-shift

measurement at or above this CTV is unlikely to be this large simply

because of measurement error. Therefore, any such measurement warrants

a noncompliance citation.

The probability that a measurement exceeds the CTV is even smaller

if i>S for any I. Furthermore, to the extent that

CVtotal is actually less than CVCTV, is

actually less than SCVCTV. This results in an even

lower probability that the critical value would be exceeded under the

null hypothesis. Consequently, if any single, full-shift measurement

equals or exceeds c, then Ho can be rejected at confidence

level of at least 95-percent. Since rejection of Ho implies

that i S for at least one value of I,

this warrants a noncompliance citation.

It should be noted that when each of several measurements is

separately compared to the CTV table, the probability that at least one

i will be large enough to force Xi

CTV when S is

greater than the probability when only a single comparison is made. For

example (still assuming S = 2 mg/m3), if CVtotal

is actually 6.6%, then the standard deviation of

is 6.6% of 2.0 mg/m3, or 0.132

mg/m3, when = S. Using

properties of the normal distribution, the probability that any single

measurement would exceed the CTV in this borderline situation is

calculated to be 0.0062. However, the

[[Page 5701]]

probability that at least one of five such measurements results in a

citation is 1--(0.9938)5 = 3.1 percent. Therefore, the

confidence level at which a citation can be issued, based on the

maximum of five measurements made in the same MMU on a given shift, is

97%.

The constant 1.64 used in calculating the CTV is a 1-tailed 95-

percent confidence coefficient and is derived from the standard normal

probability distribution. At least one commenter expressed confusion

about whether the CTV table is based on a 1-tailed or a 2-tailed

confidence coefficient. This commenter claimed that MSHA's use of a

confidence coefficient equal to 1.64 ``clearly establishes a 90%

confidence level'' rather than 95%. The commenter apparently confused

the CTV for rejecting a 1-tailed hypothesis

( S) with the pair of critical

values for rejecting a 2-tailed hypothesis

( = S) and inferring that

i simply differs from S in either direction. The

criterion for rejecting the latter hypothesis would be a measurement

either sufficiently above the applicable standard or sufficiently below

it. In testing for a difference of arbitrary direction, 1.64 would

indeed yield a pair of 90-percent confidence limits, with a 5-percent

chance of erring on either side. The purpose of the CTV table, however,

is to provide criteria for determining that the true dust concentration

strictly exceeds the applicable standard. Since such a determination

can occur only when a single, full-shift measurement is sufficiently

high, there is exactly zero probability of erroneously citing

noncompliance when a measurement falls below the lower confidence

limit. Consequently, the total probability of erroneously citing

noncompliance equals the probability that a standard normal random

variable exceeds 1.64, which is 5 percent.

One commenter alluded to testimony in the Keystone case (Keystone

v. Secretary of Labor, 16 FMSHRC 6 (Jan. 4, 1994)), suggesting that

application of the CTV to a single measurement involves an invalid

comparison of two distributions or comparison of two means. Contrary to

much of the testimony presented in that case, a determination of

noncompliance using the CTV table is based on the decision procedure

described above. It does not involve any comparison of probability

distributions or means. Nor does it involve any statistical

distribution of dust concentrations. It involves only the comparison of

an individual full-shift measurement to the applicable standard. There

is only one probability distribution involved in this comparison:

namely, the distribution of random measurement errors by which each

full-shift measurement deviates from the true dust concentration to

which the sampler unit is exposed.

Some commenters apparently misunderstood the effect of potential

weighing errors on the formula for calculating the CTV corresponding to

different applicable standards. Weight gain is estimated from the

difference between two weighings of an exposed filter capsule, adjusted

by subtracting the difference between two weighings of a control filter

capsule. Since weight gains are small compared to the total weight of

capsules being weighed, any dependence of weighing error on the

magnitude of the mass being weighed is canceled in the process of

calculating the difference. Since the standard deviation of the error

in weight gain is, therefore, essentially constant, the ratio of that

standard deviation to the dust concentration being measured decreases

with increasing dust concentration. This causes CVCTV to

decrease as the dust concentration increases. As explained above, the

CTV corresponding to S is calculated using the value of

CVCTV for dust concentrations exactly equal to S.

Consequently, the CTV corresponding to a standard of 2.0 mg/

m3 is based on a smaller value of CVCTV than the

CTV corresponding to a standard of 0.2 mg/m3.

One commenter implied that use of the CTV table relies on an

assumption that CVtotal declines at concentrations greater

than 2.0 mg/m3 (or S in general). As explained previously,

the CTV corresponding to different applicable standards is designed to

test the null hypothesis that S is not exceeded. For each applicable

standard, entries are based on the probability distribution of

observations expected under that presumption. Consequently, the

magnitude of CVtotal assumed in establishing or applying any

CTV does not decrease below the value of CVtotal calculated

for a concentration of 2.0 mg/m3, since that is the maximum

applicable standard being tested. Because the probability of wrongly

citing noncompliance is zero when S is exceeded, measurement

uncertainty at concentrations greater than S is not relevant to

noncompliance determinations. (It would, however, be relevant to

inferring compliance at a specified confidence level--i.e., to a test

of the alternative hypothesis that S is not exceeded.)

III. Validity of the CTV table

Some commenters questioned the validity of the CTV table and

challenged the formula used to calculate each CTV listed. Some objected

to the use of a normal distribution and claimed that a lognormal

distribution or nonparametric assumptions would be more appropriate.

Other commenters objected specifically to the use of a confidence

coefficient based on a standard normal probability distribution, rather

than a t-distribution. The validity of using n, rather than

(n-1), in the formula used to calculate citation threshold

values in MSHA's February 1994 notice, was also questioned. At least

one commenter contended that the formula used to generate the CTV table

is not valid for use with only one measurement.

Such comments would have some validity if the CTV table were

intended to test or estimate average concentration over some spatially

distributed region of a mine or some period greater than the single

shift during which each measurement is taken. In either case, it might

be necessary and appropriate to estimate variation in concentration

directly from the measurement samples obtained. Such an estimate could

conceivably be used in establishing a site-specific threshold value for

citation. This would, indeed, require a theoretical minimum of two

samples, or far more for valid practical applications. Estimating

variability from the samples collected would also require additional

assumptions or nonparametric methods to reflect the pattern of

variation in dust concentration between locations or shifts.

The objections raised, however, apply to a very different task from

the one for which the CTV table is designed. As explained previously,

the CTV table is not meant to test dust concentration averaged over any

period greater than the shift during which measurements were taken. Nor

is it meant to test dust concentration averaged across different

occupational locations or throughout any spatially distributed region

of the mine. Instead, the CTV table provides criteria for determining

noncompliance at individual sampling locations on individual shifts.

Neither the spatial nor temporal distribution of the dust

concentrations is germane to the intended citation criteria. Although

several measurements may be taken during a single inspection, MSHA

regards each of these measurements as relating to the dust

concentration uniquely associated on a given shift with a separate

sampling location. Each such dust concentration (i)

is the average for the atmosphere at the sampling location, accumulated

over the course of the single, full shift sampled. Since the

enforcement objective is to determine whether i > S

for any individual I, it is not necessary to estimate or assume

anything about the

[[Page 5702]]

degree to which i varies from location to location

or from shift to shift. Nor is it necessary to assume anything about

the spatial or temporal statistical distribution of

i. No such assumptions are built into the CTV

table. A normal distribution is imputed only to

, the difference between Xi and

i. Since the mean across various

i is not being estimated or tested, it is not

necessary to estimate variability among the i from

measurements taken during the inspection. MSHA emphatically agrees with

those commenters who stressed the impossibility of doing so with a

single measurement.

Those commenters who objected to MSHA's use of a normal

distribution, claiming that a lognormal distribution or nonparametric

assumptions would be more appropriate, apparently confused the

distribution of dust concentrations over time and between locations

with the distribution of errors that arise when measuring dust

concentration at a specific time and location. In other words, they

confused the distribution of i with the

distribution of . The concerns about non-

normality stem from confusion about what quantity is being estimated.

MSHA does not dispute the fact that lognormal or nonparametric

methods are often appropriate for modeling variability in occupational

dust concentrations. MSHA, however, is explicitly not claiming to

estimate any quantity beyond the average dust concentration at a

particular sampling location on a single shift. MSHA does not claim

that dust concentrations are normally distributed from shift to shift,

from occupation to occupation, or from location to location; nor is any

such assumption built into the CTV table. Since the object is not to

estimate average concentration over a range of different locations or

shifts, the statistical distribution of i is

irrelevant, and application of lognormal or nonparametric techniques in

constructing citation criteria is both unnecessary and inappropriate.

In constructing the CTV table, MSHA used a normal probability

distribution solely to represent a potential measurement error,

. This measurement error causes a

measurement Xi to deviate from i, the

actual dust concentration at a specific time and place. As

distinguished from the statistical distribution of dust concentrations,

it is generally accepted that the distribution of measurement errors

around a given concentration is normal [3]. This was explicitly

acknowledged by members of the industry panel in their Morgantown

testimony.

Similarly, criticism directed against MSHA's use of a confidence

coefficient derived from the standard normal distribution instead of

the t-distribution arises from a basic misunder standing of what is or

is not being estimated in the decision procedure. Contrary to the

remark of one commenter, use of the t-distribution is not justified as

a ``compromise'' between normal-theoretic and nonparametric

assumptions. The

t-distribution arises in statistical theory when a normally distributed

random variable is divided by an estimate of its standard deviation.

Typically it is applied to situations in which the mean and standard

deviation are estimated from the same normally distributed data,

consisting of fewer than about thirty or forty random data points. If

the estimate of standard deviation is based on more data, then the

confidence coefficient derived from the t-distribution is approximately

equal to the corresponding value derived from the standard normal

distribution. Use of the t-distribution is appropriate, for example,

when a group of normally distributed observations is ``standardized''

by subtracting the group mean from each observation and dividing the

result by the group standard deviation.

Those commenters advocating a confidence coefficient based on the

t-distribution failed to recognize that CVCTV was not

derived from the measurements that MSHA inspectors will use to test for

compliance with S. Use of the t-distribution is not appropriate when an

independently known or stipulated standard deviation is used in

comparing observations to a standard [3]. The standard deviation of

measurement errors used in constructing the CTV table is derived from

prior knowledge, rather than estimated from a few measurements taken

during an inspection. Experimental analysis has shown that

CVtotal is less than CVCTV. So long as this is

true, use of a confidence coefficient derived from the standard normal

distribution is entirely appropriate.

Contrary to the claims of some commenters, there is no valid basis

for including a so-called [n/(n-1)]1/2 ``correction factor''

in the formula for establishing a CTV. (The ``n'' in this expression

would refer to the number of measurements, if a noncompliance

determination were based on the average of several measurements.) The

theory behind such a factor does not apply when, as in the case of the

CTV table, a predetermined or maximum tolerated variability in

measurement error is used in comparing observations to a standard [3].

It would apply only if variability in measurements observed during each

inspection were somehow used to construct a CTV specific to that

inspection. The variability observed among multiple samples collected

during an MSHA inspection has little to do with the accuracy of an

individual measurement and is not used at all in constructing the CTV

table.

Although no explicit reason was given for the claim by some

commenters that the formula used to generate the CTV table is not valid

for use with a single measurement, this would follow if either: (1) the

appropriate basis for the confidence coefficient were a

t-distribution rather than a standard normal distribution; or (2) it

were necessary to multiply the CTV by [n/(n-1)]1/2, where n

is the number of measurements on which a noncompliance determination is

based. In the former case, the standard normal distribution would not

adequately approximate the t-distribution; and in the latter case, n =

1 would cause the so-called correction factor, and hence the CTV, to be

mathematically indeterminate for determinations based on a single

sample. It has already been explained, however, that neither of these

considerations are applicable to the CTV table.

Some commenters stated that a single measurement cannot accurately

be used to detect excessive dust concentrations, even if the

noncompliance determination applies only to a specific shift and

location. These commenters implied that due to random, temporary

fluctuations in dust concentration, a single measurement is inherently

unstable and misleading. Such arguments fail to differentiate a full-

shift sample from a ``grab sample,'' which is typically a sample

collected over only a few minutes or seconds and used to estimate

average conditions over an entire shift. In contrast to a grab sample,

each full-shift dust sample is collected continuously over the full

period to which the measurement applies. An 8-hour dust sample consists

of 480 1-minute grab samples, or an arbitrarily large number of even

shorter grab samples. A full-shift dust sample can be viewed as

measuring average concentration over the entire shift by averaging

together all of these shorter subsamples. Although short-term

fluctuations in dust concentration, as well as random changes in flow

rate and collection efficiency, may cause many of the subsamples to

poorly represent average concentration over the entire shift, random

short-term aberrations tend to cancel one another when the subsamples

are combined. Therefore, a

[[Page 5703]]

full-shift dust sample does not suffer from lack of sample size.

Appendix C--Risk of Erroneous Enforcement Determinations

I. What Constitutes Compliance or Noncompliance?

To simplify the following discussion, let denote the

average dust concentration to which a sampler unit is exposed on a

given shift, let S denote the applicable standard, and let X denote a

valid, full-shift measurement of . Also, let c be the CTV in

the table corresponding to S so that a citation is issued when X

c. Section 202(b)(2) of the Mine Act requires that the

average dust concentration during each shift be maintained at or below

the applicable standard wherever miners normally work or travel. This

means that, on any given shift, the average dust concentration

() at any valid sampling location must not exceed the

applicable standard (S).

Since the CTVs listed always exceed S it can happen that a full-

shift measurement (X) falls between S and c. In such instances, MSHA

will not issue a citation. This does not, however, imply that MSHA

considers the mine atmosphere sampled to have been in compliance with

the Mine Act or that cases of marginal noncompliance are tolerable.

MSHA's use of the CTVs is not motivated by any tacit acceptance of

marginal noncompliance. Rather, it is motivated by the necessity to

avoid unsustainable violations. When X falls between S and c, this

provides some evidence that > S; but the evidence is

insufficient to warrant a citation.

Although > S constitutes a violation, X greater than S

but less than the CTV does not provide compelling evidence that

> S. This is because, in a sufficiently well-controlled

mining environment, X is more likely to slightly exceed S due to

measurement error than due to > S. In fact, as demonstrated

in Appendix D, citing when X > S but X S) on the

shift and location sampled is greater than 50 percent. Use of the CTV

table is necessary in order to avoid citing in such cases.

There are two sorts of conclusions that might be drawn from the

results of a single MSHA inspection: those relating to the individual

shift sampled and those relating to some longer time period, such as

the full interval between MSHA inspections. Therefore, in evaluating

the probability of erroneous enforcement determinations, it is

essential to distinguish between (1) compliance or noncompliance with

the applicable standard on the shift sampled and (2) compliance or

noncompliance with the full requirement of the Mine Act as it applies

to every shift over a longer term, such as the period between MSHA

inspections.

If > S on some proportion of shifts, say P S on each shift and at each sampling location to

which miners in the active workings are exposed, the immediate

objective of an MSHA dust inspection can only be to determine

compliance or noncompliance for the shift and location sampled.

Therefore, MSHA does not consider a compliance or noncompliance

determination to be erroneous if it is correct with respect to the

individual shift and location but incorrect with respect to other

shifts or locations.

II. Uncertainty in the Standard-Setting Process

In response to the March, 12, 1996 MSHA/NIOSH Federal Register

notice, a commenter claimed that a noncompliance determination based on

a single, full-shift measurement could be erroneous if the applicable

standard was improperly established due to measurement errors

associated with silica analysis. It was, therefore, suggested that

uncertainty in the standard-setting process should be factored into the

risk of erroneous enforcement decisions. MSHA agrees that, like any

measurement process, the sampling and analytical method used to

quantify the silica content of a respirable dust sample in order to set

the applicable standard is subject to potential measurement errors.

Therefore, MSHA uses an analytical procedure that meets the requirement

of a NIOSH Class B analytical method. Applicable standards are set

based on results of silica analysis using the most up-to-date

laboratory equipment.

The Secretary, however, considers the accuracy of the standard-

setting process to be a separate issue from the accuracy of

noncompliance determinations based on a single-full-shift measurement,

once the applicable standard has been set. The present notice relates

only to the enforcement of the applicable standard in effect at time of

the sampling inspection. Therefore, the following discussion treats any

applicable standard in effect at the time of sampling as binding and

evaluates the risk of erroneous determinations relative to that

standard.

III. Measurement Uncertainty and Dust Concentration Variability

Variability in dust concentration refers to the differing values of

on different shifts or at different locations. For a given

value of , measurement uncertainty refers to the differing

measurement results that could arise because of different potential

measurement errors. If S, measurement error can

cause an erroneous citation. Similarly, if > S, then

measurement error can cause an erroneous failure to cite.

The ``margin of error'' separating each CTV from the corresponding

applicable standard does not eliminate the possibility of erroneous

enforcement determinations due to uncertainty in the measurement

process. A determination based on comparing X to the CTV could be

erroneous in either of two ways with respect to the individual shift

sampled: (1) the comparison could erroneously indicate noncompliance on

the shift (i.e, X c but S) or (2) the

comparison could erroneously fail to indicate noncompliance on the

shift (i.e, X > S). The margin of error built into

the CTV table reduces the probability of erroneous citations but

increases the probability of erroneous failures to cite.

MSHA recognizes that in determining how large the margin of error

should be, there is a tradeoff between the probabilities of these two

mistakes--i.e., if the chance of erroneously failing to cite is

reduced, then the chance of erroneously citing is increased, and vice

versa. MSHA has constructed the CTV. table so as to ensure that

citations will be issued only when they can be issued at a high level

of confidence. As will be shown below, doing this provides assurance

that for any given citation, is more likely than not to

actually exceed S. In contrast, if there were no margin of error,

citations more likely than not to be erroneous could occasionally be

issued. Examples of this are given in Appendix D.

[[Page 5704]]

In the discussion below, the risk of erroneous citations and

erroneous failures to cite is quantified for noncompliance

determinations based on the CTV table. To illustrate points in the

theoretical discussion, three different mining environments will be

used as examples. These environments exemplify different degrees of

dust concentration variability and dust control effectiveness. The

first example (Case 1) is based on historical mine data provided by

commenters in connection with these proceedings. The second and third

examples (Case 2 and Case 3) are hypothetical and are designed to

reflect extremely well-controlled and poorly controlled mining

environments, respectively. In these three examples, it will be assumed

that is lognormally distributed from shift to shift. This is

a standard assumption for airborne contaminants in an occupational

setting [3]. The three cases considered are characterized as follows:

----------------------------------------------------------------------------------------------------------------

Dust concentration (mg/m3)

-----------------------------------------------------------------

Case Arithmetic Arithmetic Prb {>S}

E{} SD{} mean Std. Dev. (percent)

----------------------------------------------------------------------------------------------------------------

1............................................. 1.66 0.70 1.53 1.50 25.4

2............................................. 1.20 0.24 1.18 1.22 0.4

3............................................. 2.20 1.32 1.89 1.74 45.8

----------------------------------------------------------------------------------------------------------------

In addition to the variability in dust concentrations described by

the arithmetic and geometric standard deviations of , full-

shift measurements contain a degree of uncertainty described by

CVtotal, the coefficient of variation for measurements of

the same dust concentration. In calculating the probability of

erroneous determinations for the three example cases, it will also be

assumed that the applicable standard is S = 2.0 mg/m3 and

that the coefficient of variation in full-shift measurements taken at a

given value of is:

[GRAPHIC] [TIFF OMITTED] TN31DE97.016

Where e = 9.12 g is the standard deviation

of error in weight gain, as determined from MSHA's 1995 field

investigation of measurement precision [4]; 1.38 is the MRE-equivalent

conversion factor for measurements made with an approved sampler unit;

the first quantity being squared is CVweight;

CVpump = 4.2% and CVsampler = 5%, as explained in

Appendix B.II of the joint MSHA and NIOSH notice of finding published

elsewhere in today's Federal Register.

It should be noted that the ``total'' in CVtotal refers

to total measurement uncertainty and is not meant to include the

effects of variability in dust concentration.

Because it employs a higher value for CVsampler

(reflecting variability amongst used rather than new 10-mm nylon

cyclones), this composite estimate of CVtotal is slightly

greater and perhaps slightly more realistic than that obtained directly

from MSHA's 1995 field investigation. It declines from 11.3% at dust

concentrations of 0.2 mg/m3 to no more than 6.6% at

concentrations of 2.0 mg/m3 or greater. At all dust

concentrations within this range, it falls well below the 12.8% maximum

value permitted for a method meeting the NIOSH Accuracy Criterion [5].

It is also smaller than the value, CVCTV, used to construct

the CTV table. As explained in Appendix B, this ensures that any

citation issued will be warranted at a confidence level of at least 95

percent.

To simplify the discussion below on risk of erroneous citations and

erroneous failures to cite, it is necessary to introduce some

additional notation and to focus on just one measurement collected

during each inspection.2 This could be the ``D.O.'' sample

in a MMU, or the measurement collected for a designated area. Let

= X- represent the measurement error in a valid

measurement. For reasons explained in Appendix B, is assumed

to be normally distributed with zero mean and standard deviation equal

to = CVtotal.

Consequently, X is normally distributed with mean equal to

and standard deviation equal to . This normal distribution of

X around reflects uncertainty in the measurement of a given

dust concentration. On any given shift, the probability distribution of

X is determined by the value of for that shift and sampling

location. Therefore, the probability of citation on a given shift is

conditional on and is denoted by Prb{Xc |

.}3

---------------------------------------------------------------------------

\2\ Appendix D addresses cases in which a noncompliance

determination is based on the maximum of several measurements.

\3\ A vertical bar is used to denote conditional probability.

Prb {A | B} denotes the conditional probability of event A, given

the occurrence of event B. For any events A and B,

Prb{A|B}=Prb{A and B}/Prb {B}=Prb{B|A}Prb {A}/Prb{B}

---------------------------------------------------------------------------

Since varies from shift to shift, variability in dust

concentration is represented by the probability distribution of

. Let E {} denote the expected (i.e., arithmetic

mean) dust concentration over some longer term of interest, such as the

interval between MSHA inspections; and let SD{} denote the

standard deviation of over the same period. Although the

value of on any individual shift is unknown,

Prb{Xc} can be calculated using the probability distribution

of . In particular, if the probability is known that

fulfills a specified condition, such as S or

> S, then

Prb{Xc} = Prb{Xc | S}

Prb{S}+Prb{Xc |

>S}

Prb{>S}.

Over a sufficiently long term, with respect to any particular

sampling

[[Page 5705]]

location, Prb{>S} and Prb{S} can be

identified, respectively, with the proportion of noncompliant shifts,

P, and the proportion of compliant shifts, 1-P. P is sometimes called

the noncompliance fraction and more or less defines the likelihood that

the applicable standard is or is not exceeded on the particular shift

inspected.4

---------------------------------------------------------------------------

\4\ P defines this likelihood exactly only if shifts are

randomly selected for MSHA inspection and there is no adjustment of

conditions in response to the inspection.

---------------------------------------------------------------------------

If the statistical distribution of can be adequately

represented by a probability density function, denoted f(),

then Prb{>S} and Prb{S} can also be

calculated by integrating f() over the desired range. The

probability that falls in any interval, say between a and b,

is given by:

[GRAPHIC] [TIFF OMITTED] TN31DE97.017

It follows that:

[GRAPHIC] [TIFF OMITTED] TN31DE97.018

IV. Risk of Erroneous Citation

Some commenters argued that a citation for noncompliance is

warranted only if the average dust concentration to which a miner is

exposed exceeds the applicable standard over a period of time greater

than a single shift, such as a bimonthly sampling period, a year, or a

miner's lifetime. Therefore, these commenters called it ``unfair'' to

cite individual shifts on which the applicable standard is exceeded, so

long as the average over this longer term meets the applicable

standard. For example, based on the historical sampling data provided

by a commenter and employed here as Case 1, one commenter concluded

that ``* * * there is at least a 1 in 6 or 17% probability that any

single sample can show potential overexposure [using the CTV table]

when one does not exist.'' Further, these commenters maintained that

basing citations on a single, full-shift measurement would

substantially increase the frequency of unfair citations, compared to

existing MSHA policy.

Using the notation introduced above, these commenters have confused

with E() and confounded the noncompliance fraction P

with the probability of erroneous citation. For example, the 17-percent

figure mentioned above includes all cases in which X c,

regardless of whether > S on the shift sampled. In the

discussion accompanying the data, commenters argue that since

E() is approximately 1.66 mg/m\3\, or less than 1.85 mg/m\3\

at a high confidence level, ``* * * [cases of X c] show

potential overexposure when one does not exist.'' This statement

depends on the unwarranted assumption that miners exposed to these

conditions have been exposed to similarly distributed dust

concentrations in the past and that they will be exposed to similarly

distributed concentrations in the future. These commenters' own

analysis indicates that the dust concentration has not been kept below

the standard on each shift. Therefore, a citation is warranted under

the Mine Act.

To more fully explore what is going on in Case 1, suppose, as these

commenters suggest, that dust concentrations over the period observed

are lognormally distributed from shift to shift, with E{} =

1.66 mg/m3 and a geometric standard deviation of about 1.5

mg/m3. Under this assumption, > 2.0 mg/

m3 on more than 25 percent of all shifts, and >

2.33 mg/m3 on 15 percent. These percentages pertain to

actual dust concentrations and have nothing to do with measurement

error or accuracy of an individual measurement. Therefore, a 2.0 mg/

m3 dust standard would be violated on 25 percent of all

production shifts. The applicable standard would be violated by an

amount greater than 0.33 mg/m3 on 15 percent. Since 2.33 is

the CTV for a single measurement, this 15 percent actually represents

shifts sufficiently far out of compliance that they would probably be

cited if inspected. Nevertheless, the commenters' analysis includes

such shifts in the 17 percent claimed as cases subject to erroneous or

unfair citation.

The expected value of the noncompliance fraction (P) in Case 1 is

25 percent. Therefore, close to 25 percent of all single shift

measurements made under the conditions of Case 1 would be expected to

exceed the standard. Only 17 percent of the single full-shift

measurements taken, however, exceeded the CTV and would have warranted

citations. Using the estimate of CVtotal described above, 15

percent of all single shift measurements would be expected to do so.

Therefore, contrary to the commenters' conclusion, Case 1 does not

demonstrate a high probability of erroneously identifying

overexposures. Instead, it illustrates an effect of the high confidence

level required for citation: the margin of error built into the CTV

reduces the probability of citing whatever shift happens to be selected

for inspection from about 25 percent to 15 percent. Although the

applicable standard is violated on 25 percent of the shifts, there is

only a 15 percent chance that any particular measurement meets the

citation criterion.

To correctly and unambiguously quantify the risk of ``unfair''

citations, it is necessary to identify three distinct ways of

interpreting the risk of erroneous noncompliance determinations. This

risk can be defined alternatively as:

(1) the probability of citing when the mine atmosphere sampled is

actually in compliance, Prb{Xc|S};

(2) the probability that the mine atmosphere on a shift randomly

selected for inspection is in compliance but is nevertheless cited,

Prb{S and Xc}; or

(3) the probability that a given citation is erroneous,

Prb{S|Xc}.

These three different probabilities apply to three different base

populations. Although the different interpretations of risk give rise

to quantitatively different probabilities, the expected total number of

erroneous citations, denoted N, remains constant if each

probability is multiplied by the size of the population to which it

applies. To obtain N, the first probability must be multiplied

by the number of valid measurements made when S,

the second by the total number of valid measurements, and the third by

the total number of citations issued--i.e., valid measurements for

which X c.

The CTV table limits the probability of erroneously citing defined

by the first two interpretations to a maximum of less than five

percent. However, in a

[[Page 5706]]

well-controlled mining environment, where citations are rarely

warranted, the third probability can be larger than the first two.

Since the burden of proof rests with MSHA to demonstrate noncompliance,

it is essential that deg. be kept well below 50 percent. As

will be shown by example, the use of the CTV table accomplishes this

goal.

Each of the three different probabilities related to erroneous

noncompliance determinations will now be explained in detail.

Calculations for all examples are performed under the assumptions (1)

that is lognormally distributed and (2) that is

normally distributed with mean equal to zero and standard deviation

equal to CVtotal.

1. = Prb{Xc|S}

The first risk to be considered is the probability of citing

noncompliance when the mine atmosphere sampled is actually in

compliance. This probability represents the proportion of those

measurements made when S that result in X

c. In other words,

=Prb{Xc|S} is the probability

that, due to measurement error, a citation is issued under the

condition that S. This is the probability

associated with what is commonly designated Type I error for testing

the null hypothesis: S on the shift sampled.

Essentially, is the expected (i.e., mean) probability of

citation over all those shifts sampled that are at or below the

applicable standard. The relative frequency distribution of

over those shifts is described by its probability density function,

f(). Therefore, can be calculated as follows:

[GRAPHIC] [TIFF OMITTED] TN31DE97.019

If did not vary, then would be directly related

to the confidence level at which the null hypothesis could be rejected

when X c. That confidence level, which applies to citations

issued in accordance with the CTV table, is defined as the minimum

possible value of 1-Prb{Xc|}, subject to the

restriction that S. There is a subtle but

extremely important distinction between this and 1-. Among all

those shifts on which S,

Prb{Xc|} is maximized when = S.

Therefore, the minimum possible value of 1-, arises when

= S on every shift. The resulting confidence level for

concluding > S when X c is equal to

1-Prb{Xc|=S}. For the value of CVtotal

described above (i.e., 6.6% when = S = 2.0 mg/m3),

this works out to a confidence level of 0.99, or 99%.

Although MSHA interprets the Mine Act as requiring

S on each shift at any location to which a miner in the

active workings is exposed, citations for noncompliance are intended to

apply only to the shift and location sampled. Therefore, MSHA makes no

assumption regarding the relative frequency distribution of

from shift to shift. This is consistent with the concept of defining

the confidence level according to the scenario most susceptible to an

erroneous determination under the null hypothesis. However, the

resulting confidence level for citing when X c really

applies only to the hypothetical case most susceptible to erroneous

citation.

In reality, so long as falls below S on some shifts,

will be smaller than 0.01. The further falls below

the applicable standard, and the more shifts on which this occurs, the

less likely it becomes that measurement error alone () will be

great enough to cause X c on a shift randomly selected for

inspection. For example, if S = 2.0 mg/m3, then c = 2.33 mg/

m \3\.

Therefore, if = 1.8 mg/m3, a citation would be

issued only if c-. An

0.53 mg/m3 (resulting in X 2.33 mg/

m3) amounts to a measurement error greater than 29 percent

of the true dust concentration. If the sample is valid, then the

probability of such an occurrence (given that CVtotal = 6.6%

at = 1.8 mg/m3) is less than 4 per million. This

illustrates the general point that Prb{Xc|} can be

far less than 0.01 when c|} is smaller the further

falls below S, Prb{Xc|S} depends on the

probability distribution of . This probability distribution is

expressed by the relative frequency with which assumes each

possible dust concentration at or below S. If falls

substantially below the applicable standard on many shifts, then many

of the corresponding values of Prb{X>c|} averaged into the

calculation of should be much smaller than 0.01, as shown by

the foregoing example. Consequently, in a mining environment where the

dust concentration is usually well below the applicable standard,

can reasonably be expected to fall substantially below its

maximum possible value.

The number of erroneous citations expected (N), is

obtained by first multiplying the total number of production shifts

during the period of interest by the expected proportion of these

shifts for which S. This proportion is 1 - P. The

result is the number of production shifts expected to be in compliance

at the sampling location. This must then be multiplied by to

calculate N.

In Case 1, which is based on real sampling data (submitted by

commenters), E{} is 1.66 mg/m \3\ and SD{} is 0.70

mg/m \3\. As mentioned earlier, P is expected to be 0.25 in this case.

This distribution results in a negligible probability of citing when

the mine atmosphere sampled is in compliance: = 0.00012. If

10,000 production shifts are sampled in this type of environment, 7500

of these would be expected to be in compliance at the sampling

location. Approximately one of these 7500 samples (i.e.,

7500) would be erroneously cited.

In Case 2, which is meant to represent a more controlled mining

environment, less than one percent of the shifts are expected to exceed

the standard: P = 0.0037. Furthermore, can be expected to

fall below the geometric mean of 1.18 mg/m \3\ on about half of the

shifts. Therefore, is even smaller than in the first case:

= 0.0000079. Out of 10,000 sampled shifts, 9963 would be

expected to be in compliance. Since 9963

is less than 0.1, it is unlikely that any of these shifts would be

cited erroneously.

Case 3 is meant to represent a poorly controlled mining

environment, in which E{} exceeds the applicable standard and

the coefficient of variation in shift-to-shift dust concentrations is a

relatively high 60% (i.e., 1.32 2.20). The geometric mean,

however, falls slightly below the applicable standard, so is

expected to fall below the applicable standard on more than 50% of the

shifts. The noncompliance fraction is expected to be P = 0.46. Also,

because of the high shift-to-shift variability, is not very

close to its geometric mean on most shifts, and a fairly large

percentage of shifts can be expected to experience well below

the standard. The probability of citing when the mine atmosphere is in

compliance is: = 0.00015. If 10,000 of shifts in this

environment are sampled, then 5400 of these shifts would be expected to

comply with the applicable standard at the sampling location. As in

Case 1, an erroneous citation would be expected on about one of these

shifts.

2. * = Prb{S and Xc}

The probability of erroneous citation can also be defined

unconditionally. The second way of interpreting this risk represents

the proportion of all measurements expected to result in an erroneous

citation. Let * = Prb{S and Xc}

be the probability that a shift and/or mine atmosphere randomly

selected for inspection is in compliance but, because of measurement

error, is nevertheless cited. For an erroneous

[[Page 5707]]

citation to occur, two events must take place: first, the atmosphere

sampled must be in compliance ( S); second, a

measurement error must occur of sufficient magnitude that a citation is

issued (X c). The probability that a randomly selected

shift will be in compliance is Prb{S} = 1-P. The

probability of citation, given compliance on the sampled shift, has

already been quantified above as

Prb{Xc|S} = . The probability

that both events occur is the product of these two probabilities--i.e.,

Prb{S and Xc} =

Prb{S}

Prb{Xc|S}

Therefore, *=(1-P) .

If the applicable standard is exceeded on all shifts, it is

exceeded on the shift sampled, so there is no chance of erroneously

citing that shift: i.e., P = 1, so *=(1-1)=0.

At the opposite limit, if the applicable standard is never exceeded,

then P = 0 and * = . Between these two extremes,

* decreases as the noncompliance fraction P increases, so that

* is always less than . To get the number of

erroneous citations, * is simply multiplied by the number of

shifts sampled. This always gives an identical result for N as

that obtained from multiplying the number of compliant shifts by

.

In Case 1, P = 0.25. Therefore, the probability of erroneously

citing a randomly selected shift is * =

0.75 = 0.00009, or about nine in 100,000.

If 10,000 shifts are sampled, then 10,000 *

gives the same number of erroneous citations as multiplied by

the 7500 compliant shifts expected in this case.

In the relatively well-controlled environment exemplified by Case

2, dust concentrations on most shifts generally fall well below the

standard. Only occasional excursions approaching or (rarely) exceeding

the standard occur, so P is near zero. Therefore, * is only

slightly smaller than . Since P = 0.0037, * = 0.9963

. In this environment, the chance of

erroneously citing a randomly selected shift is less than one in

100,000.

In Case 3, the noncompliance fraction is much greater: P = 46%.

Therefore, * is substantially smaller than . In this

environment the probability of erroneously citing a randomly selected

shift is * = 0.00008, or about eight in 100,000.

3. deg. = Prb{S|Xc}

Finally, the risk of an erroneous citation can be interpreted as

the probability, given a measurement of sufficient magnitude to warrant

citation (X c), that the dust concentration measured

actually complies with the standard (S). Let

deg. = Prb{S|Xc} denote this

probability, which represents the expected proportion of all citations

issued because of measurement error. If any particular citation, based

on a valid single, full-shift measurement, is selected for scrutiny,

then deg. is the probability that this citation is erroneous.

Using the definition of conditional probability:

[GRAPHIC] [TIFF OMITTED] TN31DE97.020

Prb{Xc|>S} represents the power of the citation

criterion to identify cases of noncompliance when they actually occur.

This probability is calculated as follows:

[GRAPHIC] [TIFF OMITTED] TN31DE97.021

When the distribution of dust concentrations is such that the

applicable standard is rarely exceeded (i.e., when P is near zero), the

denominator in the expression for deg. namely

[GRAPHIC] [TIFF OMITTED] TN31DE97.022

is only slightly greater than the numerator, *. This implies

that deg. is not constrained to be smaller than or

*. Since this situation arises in environments where the

applicable standard is rarely exceeded, such citations will not often

be issued. However, when one is issued, the probability that it is

erroneous can exceed .

For example, in the relatively well-controlled environment

exemplified by Case 2, * is 0.00000788, P is 0.00370, and

Prb{Xc|>S} = 0.133. Therefore, in this example,

deg. = 0.0158, or about 1.6 percent. That is to say, 1.6

percent of the citations issued under these circumstances will be

erroneous. This is considerably greater than , which was

earlier shown to equal only 0.00079 percent. However the expected

proportion of measurements resulting in citation, given by

Prb{Xc}, is only 0.000498, or 0.050%. Therefore, out of

[[Page 5708]]

10,000 shifts sampled, it is expected that only five would be cited.

Since on average only 1.6% of these five citations would be erroneous,

it is unlikely that the 10,000 samples would result in any erroneous

citations.

Case 2 represents an environment in which the noncompliance

fraction is less than one percent. In contrast, the noncompliance

fraction in Case 3 is nearly 50%: P = 0.458. For this case, =

0.000147, * = 0.0000799, and deg. = 0.000227. The

calculated value of Prb{Xc} is 0.3513, so approximately 35

percent of all measurements would result in citation. Only about 0.027%

of these citations, however, would be erroneous. Therefore, out of

10,000 shifts sampled in such an environment, 3513 citations could be

expected; and only about one of these citations

(3513 deg.) would be expected to be erroneous.

In Case 2, the probability ( deg.) that a given citation

is erroneous is relatively high (though low enough to sustain a

citation), but the probability of citing noncompliance in such an

environment is very low. In Case 3, the probability of citation is more

than 700 times higher, but deg. is commensurately lower than

in Case 2. Comparison of Cases 2 and 3 illustrates the general

principle: as the noncompliance fraction P increases, the probability

of citation increases but the probability that a given citation is

erroneous decreases.

It is important to note that even in the well-controlled

environment of Case 2, the probability that a given citation is

erroneous ( deg.) remains substantially below five percent and

far below 50 percent. Although environments even more well controlled

could give rise to somewhat greater values of deg., the

probability of citing in such environments would be even smaller than

the probability in Case 2. If a citation is issued because X > c, then

the probability that > S is simply 1 - deg.. This

shows that in any particular instance where a citation based on a

single, full-shift measurement is reasonably likely to be issued

according to the CTV table, there would be compelling evidence that

> S.

V. Risk of Erroneous Failure to Cite

Use of the CTV implies that citations will be issued only when they

can be issued with high confidence that the applicable standard has

actually been exceeded on the shift sampled. On the other hand, failure

to meet or exceed the CTV does not in itself imply compliance at a

similarly high confidence level--even on the shift sampled, let alone

continuously over any longer term. Because of limited resources, MSHA

inspections are relatively infrequent and serve only to identify

instances in which the rest of the dust control program has been

ineffective. They cannot be relied upon to ensure continuous

compliance.

It should be remembered, however, that MSHA does not rely

exclusively on sampling by inspectors to ensure compliance. The MSHA

inspection is only one element of the Agency's comprehensive health

protection program, which includes mandatory implementation and

maintenance by operators of effective dust control methods to control

dust levels where miners normally work or travel. It also provides for

periodic evaluation by mine operators of the quality of mine air and of

the effectiveness of the operator's dust control system through

operator bimonthly sampling. If they are not detected during an MSHA

inspection, poorly controlled environments, which are out of compliance

with the dust standard in a substantial fraction of instances, are

likely to be detected during some other phase of the MSHA's enforcement

program.

It should also be remembered that MSHA's new enforcement policy

eliminates an important source of sampling bias due to averaging, as

explained in Appendix A. Under the existing policy, measurements made

at the dustiest occupational locations or during the dustiest shifts

sampled are diluted by averaging them with measurements made under less

dusty conditions. As shown by the SIP data, this practice has

frequently caused failures to cite clear cases of excessive dust

concentration.

1. = Prb{X>S}

The complement of power, the probability of detecting cases of

noncompliance when they occur, is the probability of erroneously

failing to detect such cases. Let = Prb{X>S} be

the probability that a citation will not be issued when the true dust

concentration being measured exceeds the standard. This is the

probability of what is commonly called Type II error for testing the

null hypothesis that S. Since = 1 -

Prb{Xc|>S}, the power of the citation criterion,

formulated earlier as Prb{Xc|>S}, can be used to

calculate . The expected number of erroneous failures to cite,

N is obtained by multiplying by the number of shifts

for which > S.

It is true that due to the high confidence level required for

citation, is greater than it would be if a citation were

issued whenever X > S. In fact, setting the CTV to any value greater

than S results in Prb{X} potentially greater than 50

percent when a single dust concentration exceeding the standard is

being measured. For example, if = 2.12 mg/m3 and S

= 2.0 mg/m3, then the CTV is c = 2.33 mg/m3.

Since the probability distribution for X is centered on , any

individual measurement is more likely to fall below the CTV than to

exceed it. The probability of erroneously failing to cite in this

instance, based only on a single measurement, would be

Prb{X=2.12} = 93 percent.

Citing in accordance with the CTV table does not, however,

necessarily result in > 50%. When more than one measurement

is made during a single shift in the same general area of a mine, such

as in the same MMU, the dust concentrations are correlated. This

increases the chances that if exceeds the standard at one of

the sampled locations, at least one of the measurements will meet the

citation criteria. More importantly for the present discussion,

however, the value of depends on the distribution of

even when only a single measurement is considered on each

shift.

This is because the magnitude of depends on the average

magnitude of Prb{X} over all those instances in which

> S. Although Prb{X} exceeds 50 percent when

> c. Poorly

controlled environments are likely to experience a significant number

of shifts during which exceeds not only S but also the CTV.

If these shifts ``outweigh'' those shifts on which S

c, then this will result in > S, Prb{X} exceeds

50% only when falls between S and c. In contrast, the range

of potential values of >c is essentially unlimited, and

Prb{X} approaches zero as increases. Therefore,

is less than 50% whenever the distribution of is

such that Prb{>c} > Prb{S c}. In a poorly

controlled environment, is more likely to exceed the CTV than

to fall into the relatively narrow interval between S and the CTV.

For example, in Case 1 the probability that exceeds c =

2.33 is 14.9 percent, whereas the probability that falls

between S and c is only P - 14.9 = 10.5 percent. Therefore, in this

environment, the probability of erroneously failing to cite an instance

of > S works out to be somewhat less than 50 percent:

= 1 - Prb{Xc|>S} = 0.404, or 40.4%.

For worse offenders, is considerably smaller. In Case 3,

Prb{>c} = 35.2%, whereas Prb{Sc} is

10.6%. In this case, even though dust concentrations below the

applicable standard are

[[Page 5709]]

expected on a majority of shifts (as indicated by the geometric mean),

is calculated to be only 23.3%. Stated another way, if MSHA

were to select 10,000 shifts in this environment, an expected 4580 of

those shifts would be out of compliance. It is expected that on 76.7%

of those 4580 shifts a single measurement would be sufficiently large

to warrant citation.

There are inherent tradeoffs, not only between and

, but also between and the probability that a given

citation is erroneous, deg. =

Prb{S|Xc}. Decreasing the CTV in order

to reduce forces both and deg. to

increase. Even if remains below 50 percent, the effect on

deg. can be so great as to render some citations clearly

unsustainable. In particular, setting the CTV at or near S could result

in citations more likely than not to be erroneous. Circumstances in

which this can occur are discussed in Appendix D. Use of the CTV, on

the other hand, ensures that any given citation based on X

c is more likely than not to represent a case of actual noncompliance

(i.e., > S).

Failure to issue a citation based on a single, full-shift

measurement collected during an MSHA inspection does not imply failure

to detect and correct a noncompliant condition in the context of MSHA's

entire enforcement program. Those commenters expressing concern over

the potential magnitude of have largely ignored other means

MSHA uses to protect miners from excessive dust concentrations relative

to the longer term. As stated earlier in this notice, MSHA's health

protection program provides for the implementation and maintenance by

mine operators of effective methods to control dust concentrations

where miners normally work or travel, as well as for periodic

evaluation of the quality of mine air to which miners may be exposed

and the effectiveness of the operator's dust control program through

operator bimonthly sampling. Furthermore, MSHA intends to continue its

long-standing practice of collecting additional measurements when the

standard is exceeded by an amount insufficient to warrant citation at a

high confidence level.

VI. Summary and Conclusions

Use of the CTV table is based on MSHA's need for sufficient

evidence to issue a citation and show, by a preponderance of the

evidence, that a violation occurred. The burden rests with MSHA to show

that the applicable standard has in fact been violated on the

particular shift cited. Accordingly, the CTV table is designed so that

the risk of erroneously not citing is subordinated to the risk of

erroneously issuing a citation. However, the probability of erroneously

failing to cite a case of noncompliance at a given sampling location is

less than 50 percent when the applicable standard is exceeded on a

significant proportion of shifts at that location.

Three cases were used to illustrate the risk of erroneous

enforcement determinations over a broad range of environmental

conditions. The results calculated for each of the three cases

considered are summarized in the following table.

--------------------------------------------------------------------------------------------------------------------------------------------------------

Probability (percent) Average number of

------------------------------------------------------------------------------------------ erroneous determinations

(per 10,000 sampled

Case shifts)

Prb{X>S} Prb{Xc} * deg. -------------------------

N N

--------------------------------------------------------------------------------------------------------------------------------------------------------

1................................... 25.51 15.14 0.0121 0.00903 0.060 40.4 0.9 1,026

2................................... 0.53 0.05 .000791 .000788 1.581 86.7 .1 32

3................................... 45.69 35.17 .0147 .00799 0.0227 23.3 .8 1,067

--------------------------------------------------------------------------------------------------------------------------------------------------------

Based on this analysis, it can be concluded that application of the

CTV table provides ample protection against erroneous citations. The

probability () of issuing a citation when the mine atmosphere

sampled is actually in compliance is constrained to fall below a

maximum of five percent. This maximum defines the

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