Maria Elosu v. Middlefork Ranch Incorporated

FOR PUBLICATION

UNITED STATES COURT OF APPEALS

FOR THE NINTH CIRCUIT

MARIA FERNANDA ELOSU, No. 21-35309

Individual; ROBERT LOUIS BRACE,

Plaintiffs-Appellants, D.C. No.

1:19-cv-00267-

v. DCN

MIDDLEFORK RANCH

INCORPORATED, an Idaho OPINION

Corporation,

Defendant-Appellee.

Appeal from the United States District Court

for the District of Idaho

David C. Nye, Chief District Judge, Presiding

Argued and Submitted December 8, 2021

Pasadena, California

Filed February 23, 2022

Before: Carlos T. Bea and Kenneth K. Lee, Circuit Judges,

and Richard D. Bennett, * District Judge.

Opinion by Judge Bennett

*

The Honorable Richard D. Bennett, United States District Judge

for the District of Maryland, sitting by designation.

2 ELOSU V. MIDDLEFORK RANCH

SUMMARY **

Expert Testimony

The panel reversed the district court’s partial grant of

Middlefork Ranch Inc.’s motion to exclude expert testimony

and the subsequent entry of summary judgment in a diversity

negligence action, and remanded for further proceedings.

Plaintiffs were the owners of a vacation cabin in Idaho

that burned. An expert report prepared by fire investigator

Michael Koster hypothesized that an open-flame pilot light

ignited combustible vapors from an oil stain on a wooden

deck and sparked the fire that burned the entire structure to

the ground. The district court excluded Koster’s testimony,

finding that the substance of his opinion was speculative,

uncertain and contradicted by multiple eyewitness accounts.

The panel held that the district court improperly assumed

a factfinding role in this case. Although a court may screen

an expert opinion for reliability, and may reject testimony

that is wholly speculative, it may not weigh the expert’s

conclusions or assume a factfinding role. In the plain text of

its opinion, the district court took issue only with the expert’s

ultimate conclusions. In its findings, the district court

disregarded much of the expert’s scientific analysis, weighed

the evidence on record, and demanded corroboration –

factfinding steps that exceeded the court’s gatekeeping role.

**

This summary constitutes no part of the opinion of the court. It

has been prepared by court staff for the convenience of the reader.

ELOSU V. MIDDLEFORK RANCH 3

COUNSEL

Patrick C. Bageant (argued), Hollystone Law, Boise, Idaho,

for Plaintiffs-Appellants.

Gerald Kobluk (argued), KSB Litigation P.S., Spokane,

Washington, for Defendant-Appellee.

OPINION

BENNETT, Circuit Judge:

On July 20, 2017, a fire swept through a cabin in the

Idaho wilderness. Nobody was at home, and neither

residents, neighbors, nor first responders saw the cabin catch

fire. Appellants Maria Elosu and Robert Brace contend that

the fire was caused by a negligent employee of Appellee

Middlefork Ranch, Inc. (“Middlefork”), the homeowners’

association which governs the cabin. Their case rises and

falls on an expert report prepared by fire investigator

Michael Koster, who hypothesized that an open-flame pilot

light on the northern end of the cabin ignited combustible

vapors from an excessive oil stain that had been applied to

the wooden deck the previous day—sparking a fast-moving

conflagration that swept across the deck and burned the

entire structure to the ground.

This appeal arises from an order of the United States

District Court for the District of Idaho excluding Koster’s

testimony—and precluding Appellants’ case on the critical

element of causation, the sole triable issue that remained in

this litigation. The district court did not hold that Koster’s

methodology was unreliable or that he was not qualified to

render an expert opinion in the field of fire investigation.

4 ELOSU V. MIDDLEFORK RANCH

Indeed, the parties stipulated to his qualifications and

methodology. 1 Instead, the court took issue with Koster’s

“ultimate conclusions,” finding that the substance of his

opinion was speculative, uncertain, and contradicted by

multiple eyewitness accounts.

District courts have a longstanding responsibility to

screen expert testimony, and to prevent unfounded or

unreliable opinions from contaminating a jury trial. See

generally Daubert v. Merrell Dow Pharm., Inc., 509 U.S.

579, 586 (1993). However, this Court has cautioned that the

district court is “a gatekeeper, not a fact finder.” Primiano v.

Cook, 598 F.3d 558, 568 (9th Cir. 2010) (quoting United

States v. Sandoval-Mendoza, 472 F.3d 645, 654 (9th Cir.

2006)). Although a district court may screen an expert

opinion for reliability, and may reject testimony that is

wholly speculative, it may not weigh the expert’s

conclusions or assume a factfinding role. We are compelled

to find that the district court assumed such a role in this case.

In the plain text of its opinion, the district court took issue

only with Koster’s “ultimate conclusions.” In its findings,

the court disregarded much of the expert’s scientific

analysis, weighed the evidence on record, and demanded

corroboration—factfinding steps that exceed the court’s

gatekeeping role. Accordingly, we reverse the judgment of

1

Those qualifications are extensive. Koster is a Fire/Arson

Investigator employed by Reliant Investigations, Inc., and has been

personally involved with 2400 fire investigations in 22 years of service

in the field. He has completed six courses and eleven training sessions

related to fire investigation and forensics, received multiple relevant

certifications, attended several conferences related to fire and arson

investigation, taught twenty-three courses, and been certified as an

expert witness in several prior cases.

ELOSU V. MIDDLEFORK RANCH 5

the district court and remand for further proceedings

consistent with this opinion.

BACKGROUND

Maria Elosu and Robert Brace, husband and wife, were

the owners and seasonal residents of Cabin 16, a luxury

vacation cabin located in the remote Valley County, Idaho,

and governed by Middlefork Ranch, Inc. Cabin 16 was a flat,

one-story building with a 500-square-foot wooden deck that

wrapped around the north, east, and south sides of the

structure. The northern end of the deck featured a propane-

fueled refrigerator atop an open-flame pilot light, with a

propane supply provided and managed by Middlefork. The

east side of the deck held various items of wooden

furniture—an old sofa, a teak picnic table, and benches. 2

On the day before the fire, Brace power-washed and

stained the deck with Penofin-brand oil. Penofin oil is highly

flammable: Penofin oil cans come with detailed disposal

instructions and a warning label that specifically alerts the

user to the potential for “spontaneous combustion.”

Appellants’ chemical expert Douglas Byron concluded in his

report that Penofin oil is approximately as incendiary as

charcoal starter fluid—with a higher density than air and a

tendency to self-heat. When properly applied, Penofin oil

becomes dry to the touch after four hours, “serviceable” after

twelve, and fully cured after four to seven days. Based on the

application instructions, which dictate that one gallon should

be used for every 300 square feet of application, a 500-

2

Elosu also smokes cigarettes. The night before the fire, Elosu and

a guest smoked a cigarette on the deck, and ashes were found on the deck

the morning of the fire. Elosu smoked another cigarette “on the ground”

outside the following morning.

6 ELOSU V. MIDDLEFORK RANCH

square-foot deck should be treated with approximately one-

and-a-half gallons of Penofin oil. Both parties acknowledge

that Brace applied as much as two-and-a-half gallons that

day.

On July 20, 2017, Appellant Brace left Cabin 16 prior to

8:00 a.m. He claims that he noticed sticky spots on the

wooden deck where the Penofin oil had failed to dry. Brace

provided rags for Elosu and their child to clean the deck,

instructing them to dispose of them in the fire pit. 3 Elosu

allegedly found the deck too sticky to clean. Thereafter, the

parties concur that a Middlefork employee visited Cabin 16

to check the propane levels, determined that the propane was

out or low, and elected to refill the propane and reignite the

pilot light. The parties also acknowledge that this employee

confirmed that the deck below the refrigerator was dry

before he lit the pilot light. Appellants further claim that the

deck remained sticky with oil, that “[Middlefork’s]

employee was standing in wet oil next to the refrigerator”

when he lit the flame, and that Elosu expressed concern that

the oil could ignite.

Around 4:00 p.m., while Elosu and her child were away,

Cabin 16 caught fire. The first people to notice the fire were

a group of contractors working on a neighboring cabin,

including Kenny Pyle, Regee Rauch, and Greg Gamez.

These contractors did not see the fire ignite and arrived well

after the conflagration had spread. When they arrived, the

eastern deck was filled with flames, localized specifically

near the southeast corner. Rauch ran onto the north deck to

3

The parties dispute whether these rags were used and how they

were disposed of. One rag was entered into evidence, while two remain

unaccounted for. Appellants allege that they disposed of the rags in the

firepit.

ELOSU V. MIDDLEFORK RANCH 7

bang on the windows and draw the attention of anyone

inside. Pyle attempted to extinguish the fire with a garden

hose, but retreated when the fire spread and engulfed the

entire structure. The cabin burned to the ground and was “a

complete and total loss.”

Each party’s insurance company conducted initial

examinations of the scene: Appellants’ insurer sent Shane

Hargrove four days after the fire, while Middlefork’s insurer

sent Glen Johnson two months later. Hargrove and Johnson

each investigated the scene individually, before conducting

joint interviews of the eyewitnesses, each of whom

confirmed that there had not been a fire on the north-facing

deck when they arrived. 4 Johnson concluded that the fire had

originated on the southeast deck, and that it could have been

caused by discarded oil-soaked rags or a mop head. But

Johnson had no direct evidence of his theory, Hargrove

characterized the fire site as a “black hole,” and neither could

offer an opinion about the specific “source of ignition” that

rose to the level of “more probable than not.”

Elosu and Brace retained three additional experts to

examine the fire site: Michael Koster, a fire investigator,

Richard Mumper, a mechanical engineer, and Douglas

Byron, a chemist. Mumper evaluated the remains of the

cabin and confirmed that there was no evidence of a

mechanical breakdown or another point of ignition. Byron

conducted a chemical analysis of Penofin oil in light of the

weather conditions on the day of the fire, and confirmed that

the Xylene vapors released by evaporating Penofin could

4

Middlefork exhaustively contends that Elosu and Brace pressured

these witnesses to change their testimony and confirm Appellants’ theory

that the fire originated on the north deck. These contentions were not

addressed by the district court and are not before us on appeal.

8 ELOSU V. MIDDLEFORK RANCH

cause an explosion upon contact with an open flame. He also

determined that there was “no evidence or indication of

spontaneous combustion,” as witnesses had not smelled an

acrid odor, and video taken of the fire did not display the off-

white smoke characteristic of this phenomenon.

Michael Koster, a fire investigator employed by Reliant

Investigations, Inc., was the only expert to offer a specific

theory as to the cause and origin of the fire. 5 Koster

personally examined the remains of Cabin 16 on May 27,

2018, evaluating the cabin’s wiring, structure, insulation,

and propane system to rule out external issues. 6 He retrieved

several five-gallon buckets of Penofin oil from a shed on the

property—one of which was half empty. He compared the

scene on the ground to photos of the cabin taken before the

fire to calculate “mass loss,” flame length, and rates of

exposure—concluding that the north end of the cabin had

seen “the most significant amount of mass loss.” He also

spoke with Brace to confirm his story of events, and

reviewed tapes of witness interviews from the previous

investigations.

Koster reported his findings on January 10, 2020. In this

report, Koster reviewed literature on volatile liquids and

lumber fires, and applied professional standards governing

fire and explosion investigations, see National Fire

Protection Association, NFPA 921: Guide for Fire and

5

Mumper concurred with Koster’s theory as to the cause and origin

of the fire. However, the court granted Middlefork’s motion to exclude

this portion of Mumper’s report, reasoning that “Mumper is a mechanical

engineer, and appears to have been retained to opine on the mechanical

issues in this case, not the origins of the fire.” Elosu and Brace do not

challenge this ruling on appeal.

6

Koster’s examination was delayed by the winter frost.

ELOSU V. MIDDLEFORK RANCH 9

Explosion Investigations (2017 ed.) (“NFPA 921”), to

conclude that Cabin 16 had likely been destroyed by a fast-

moving vapor fire that was ignited by the refrigerator pilot

light. His hypothesis was that the excess Penofin oil Brace

applied to the deck had pooled beneath the floorboards,

evaporated during the heat of the following day, and ignited

upon encountering the open flame. The resulting

conflagration had swept through the deck, ignited the oil

stains and the furniture, and eventually consumed the entire

cabin.

Koster reviewed Johnson’s report, and opined that it had

failed to comply with the NFPA 921 standards. Koster

challenged Johnson’s conclusion that the fire was caused by

discarded rags or a mop head, as “no remnants of cotton fiber

materials or rags were found . . . within his suspected area of

origination.” In a subsequent deposition, Koster also noted

that his opinion was consistent with the testimony of the

eyewitnesses, as the fire may have spread to the southeast

deck. Nevertheless, Koster also acknowledged that none of

the witnesses had substantiated his theory that the fire had

originated on the north side of the deck.

Appellants filed suit against Middlefork on July 11,

2019. Consistent with Koster’s testimony, Appellants

alleged that Middlefork’s employee was negligent in

lighting the pilot light despite the fire hazard posed by the

cabin’s oil-stained deck. The case was reassigned to the

United States District Court for the Western District of

Washington to alleviate the District of Idaho’s heavy

caseload. One year later, a District Judge in Washington

denied the parties’ cross-motions for summary judgment and

transferred the case back to the District of Idaho for trial. On

October 23, 2020, Middlefork filed a Motion to Exclude

Expert Testimony, challenging all three of Appellants’

10 ELOSU V. MIDDLEFORK RANCH

experts. The district court granted this motion in part,

excluding Koster’s expert report and limiting the

admissibility of Mumper’s report. Thereafter, the parties

jointly stipulated to summary judgment, concurring that the

exclusion of Koster’s report had “eliminated a genuine

dispute of material fact as to the causation element of

[Appellants’] negligence claim.”

This appeal followed.

STANDARD OF REVIEW

“We review evidentiary rulings for abuse of discretion

and reverse if the exercise of discretion is both erroneous and

prejudicial.” City of Pomona v. SQM N. Am. Corp., 750 F.3d

1036, 1043 (9th Cir. 2014) (citing Nev. Dep’t of Corr. v.

Greene, 648 F.3d 1014, 1018 (9th Cir. 2011)). This standard

applies on appeal from a motion to exclude expert testimony.

Kennedy v. Collagen Corp., 161 F.3d 1226, 1227 (9th Cir.

1998); Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137,

152 (1999). The district court’s underlying factual

determinations are reviewed for clear error. United States v.

Lukashov, 694 F.3d 1107, 1114 (9th Cir. 2012).

ANALYSIS

This case concerns the scope of a district court’s

discretion to exclude expert testimony that it deems

unsupported by the record. Rule 702 of the Federal Rules of

Evidence tasks a district court judge with “ensuring that an

expert’s testimony both rests on a reliable foundation and is

relevant to the task at hand.” Daubert, 509 U.S. at 597; see

also Daubert v. Merrell Dow Pharms., Inc. (Daubert II),

43 F.3d 1311, 1313 (9th Cir. 1995). Rule 702 provides that

expert testimony is admissible if:

ELOSU V. MIDDLEFORK RANCH 11

(1) the witness is sufficiently qualified as an

expert by knowledge, skill, experience,

training, or education; (2) the scientific,

technical, or other specialized knowledge

will help the trier of fact to understand the

evidence or to determine a fact in issue;

(3) the testimony is based on sufficient facts

or data; (4) the testimony is the product of

reliable principles and methods; and (5) the

expert has reliably applied the relevant

principles and methods to the facts of the

case.

City of Pomona, 750 F.3d at 1043 (citing Fed. R. Evid. 702).

This appeal brings the third element of Rule 702 into

tension with the rest. The parties concur that Koster is

qualified as an expert in fire investigation, that his testimony

is the product of reliable methodology, and that his report is

relevant to the critical fact at issue—the cause and origin of

the fire. Neither party contested these elements below, and

neither party argues them on appeal. Instead, this appeal

exclusively turns on whether Koster’s testimony was “based

on sufficient facts or data” as required by Rule 702. The

district court concluded that Koster’s report is too

speculative, that his conclusion conflicted with the

contractors’ testimony, and that he relied too heavily on the

testimony of the plaintiffs. This was an abuse of discretion,

as the district court assumed a factfinding role in its analysis.

These concerns speak to corroboration, not foundation, and

are properly addressed through impeachment before a jury

at trial—not exclusion by a district judge at the admissibility

stage.

12 ELOSU V. MIDDLEFORK RANCH

I. The District Court’s Gatekeeping Role

As construed in Daubert v. Merrell Dow

Pharmaceuticals, Inc., Rule 702 tasks a district judge with

“ensuring that an expert’s testimony both rests on a reliable

foundation and is relevant to the task at hand.” 509 U.S.

at 597. “Expert opinion testimony is relevant if the

knowledge underlying it has a valid connection to the

pertinent inquiry. And it is reliable if the knowledge

underlying it has a reliable basis in the knowledge and

experience of the relevant discipline.” Alaska Rent-A-Car,

Inc. v. Avis Budget Grp., Inc., 738 F.3d 960, 969 (9th Cir.

2013) (quoting Primiano, 598 F.3d at 565). To evaluate

reliability, the district court “must assess the expert’s

reasoning or methodology, using as appropriate criteria such

as testability, publication in peer-reviewed literature, known

or potential error rate, and general acceptance.” City of

Pomona, 750 F.3d at 1044. These factors are nonexclusive,

and “the trial court has discretion to decide how to test an

expert’s reliability . . . based on the particular circumstances

of the particular case.” Id. (quoting Primiano, 598 F.3d at

564).

Ultimately, “the test under Daubert is not the correctness

of the expert's conclusions but the soundness of his

methodology.” Daubert II, 43 F.3d at 1318. The court is “a

gatekeeper, not a fact finder.” Primiano v. Cook, 598 F.3d

558, 568 (9th Cir. 2010) (quoting Sandoval-Mendoza,

472 F.3d at 654). Accordingly, “[t]he district court is not

tasked with deciding whether the expert is right or wrong,

just whether his testimony has substance such that it would

be helpful to a jury.” Alaska Rent-A-Car, Inc., 738 F.3d

at 969–70. If the proposed testimony meets the thresholds of

relevance and reliability, its proponent is “entitled to have

the jury decide upon [its] credibility, rather than the judge.”

ELOSU V. MIDDLEFORK RANCH 13

Sandoval-Mendoza, 472 F.3d at 656. “Challenges that go to

the weight of the evidence are within the province of a fact

finder, not a trial court judge. A district court should not

make credibility determinations that are reserved for the

jury.” City of Pomona, 750 F.3d at 1044; accord Alaska

Rent-A-Car, Inc., 738 F.3d at 969 (“[T]he judge is supposed

to screen the jury from unreliable nonsense opinions, but not

exclude opinions merely because they are impeachable.”).

This Court has previously noted that “[s]haky but admissible

evidence is to be attacked by cross examination, contrary

evidence, and attention to the burden of proof, not

exclusion.” Primiano, 598 F.3d at 564.

An expert’s specialized knowledge and experience can

serve as the requisite “facts or data” on which they render an

opinion. In Primiano v. Cook, a patient who had received an

elbow replacement sued the manufacturer of her prosthesis,

alleging that defects in the device’s polyethylene

components had caused joint problems that required several

additional surgeries to correct. 598 F.3d at 562. The patient

adduced testimony by Arnold-Peter Weiss, M.D., who

opined that polyethylene prostheses normally last between

five and fifteen years, and that it was unlikely plaintiff’s joint

issues were caused by “overuse, medical malpractice . . . or

other factors external to the device.” Id. at 562–63. The

district court granted the defendants’ motion to exclude this

testimony under Daubert, observing that Dr. Weiss had

failed to base his assumptions on an “objective source,” as

he had neither examined the plaintiff personally nor cited

any peer-reviewed publications that corroborated his

opinion. Id. at 563, 567.

This Court reversed, holding that the district court’s

concerns spoke to weight, not reliability. Id. at 568. We

placed particular emphasis on Dr. Weiss’s experience and

14 ELOSU V. MIDDLEFORK RANCH

qualifications, the nature of medical testimony, and the

unusual issue at hand. Id. at 566. As “medical knowledge is

often uncertain” due to the complexity of the human body

and the novelty of emerging medical issues, we reasoned that

“physicians must use their knowledge and experience as a

basis for weighing known factors along with the inevitable

uncertainties” to make “a sound judgment” in each case. Id.

at 565–66 (quoting Sandoval-Mendoza, 472 F.3d at 655).

Dr. Weiss’s “extensive, relevant experience” with the

implantation or revision of prosthetic elbows qualified him

to make such a judgment and provided a sufficient

foundation for his testimony. Id. “Given that the judge is ‘a

gatekeeper, not a fact finder,’ the gate could not be closed to

this relevant opinion offered with sufficient foundation by

one qualified to give it.” Id. at 568 (citation omitted).

Relatedly, the requirement of “sufficient facts or data”

does not preclude an expert from making projections based

on reliable methodology. In Alaska Rent-A-Car, Inc. v. Avis

Budget Grp., Inc., featuring a breach of an exclusivity

agreement by a nationwide rent-a-car chain, the plaintiff

rental car company offered expert testimony to prove

damages. 738 F.3d at 967. This testimony required the expert

to “address a hypothetical world that never existed,”

extrapolating market projections based on similarly situated

rental companies to estimate the plaintiff’s lost profits. Id.

at 968. The defendant moved to exclude the expert’s opinion

as speculative, highlighting various market differences that

undercut his conclusions. Id. at 968–69. Rejecting this

assertion, the district court admitted this testimony, and we

affirmed, observing that Avis had not challenged the

expert’s credentials, qualifications, or methodology—only

the accuracy and credibility of his final projections. Id. at

970. Each of the defendant’s countervailing considerations

were appropriate matters for impeachment, not

ELOSU V. MIDDLEFORK RANCH 15

admissibility. Id. at 969–70 (“[T]he judge is supposed to

screen the jury from unreliable nonsense opinions, but not

exclude opinions merely because they are impeachable.”).

Finally, while a court may reject wholly speculative or

unfounded testimony, it abuses its discretion if it overlooks

relevant data submitted as the foundation of an expert’s

remarks. In Kennedy v. Collagen Corp., the plaintiff sued the

manufacturer of Zyderm, a facial smoothing product that

required subcutaneous injections, alleging that its

application had caused her to develop systemic lupus

erythematosus (“SLE”), “a debilitating and incurable

autoimmune disease.” 161 F.3d at 1227. The plaintiff

offered testimony by Dr. Joseph Spindler, who opined that

there was a link between Zyderm and SLE, relying on “peer-

reviewed publications and clinical studies” that suggested

Zyderm produced autoimmune antibodies. Id. at 1228. The

district court excluded this testimony, noting that no

epidemiological studies had confirmed a link between

Zyderm and SLE, and that there was too great an “analytical

gap” between the data presented in the case and the expert’s

conclusion. Id. at 1229–30. And we reversed, observing that

“[t]he court did not consider all of the data relied upon by

Dr. Spindler, namely, studies by the defendant and others

finding that Zyderm can induce autoimmune reactions.” Id.

at 1230. Accordingly, “the gap was of the district court’s

making.” Id.

The common thread running through these cases is that

Rule 702’s “sufficient facts or data” element requires

foundation, not corroboration. Consistent with the court’s

gatekeeping function, Rule 702 instructs a district court

judge to determine whether an expert “had sufficient factual

grounds on which to draw conclusions.” Damon v. Sun Co.,

Inc., 87 F.3d 1467, 1475 (1st Cir. 1996); Kennedy, 161 F.3d

16 ELOSU V. MIDDLEFORK RANCH

at 1228 (“[T]he focus of the inquiry envisioned by Rule 702

must be on the principles and methodology underlying an

expert’s testimony, not on the conclusions.”); see also City

of Pomona, 750 F.3d at 1049 (“A factual dispute is best

settled by a battle of the experts before the fact finder, not by

judicial fiat.”). Although “[a] court may conclude that there

is simply too great an analytical gap between the data and

the opinion proffered,” Gen. Elec. Co. v. Joiner, 522 U.S.

136, 146 (1997), Rule 702 does not license a court to engage

in freeform factfinding, to select between competing

versions of the evidence, or to determine the veracity of the

expert’s conclusions at the admissibility stage.

This is consistent with the basic function of expert

testimony: to help the trier of fact understand highly

specialized issues that are not within common experience.

See Kumho Tire, 526 U.S. at 148–49, 156–57. Experts

working in specialized, scientific, and uncertain fields

regularly “extrapolate from existing data” and generate

novel hypotheses about complex issues. Joiner, 522 U.S.

at 146. For this reason, “an expert is permitted wide latitude

to offer opinions, including those that are not based on

firsthand knowledge or observation.” Daubert, 509 U.S.

at 592–93. The court’s role is to determine “the scientific

validity” of an expert’s “principles and methodology,” not to

determine whether their hypothesis is correct, or to evaluate

whether it is corroborated by other evidence on the record.

Id. at 594–95. That is for the litigants to argue, and for the

jury to decide.

II. Evaluating Koster’s Proposed Expert Testimony

Applying the principles articulated above, we must

determine whether the district court’s analysis of Koster’s

report exceeded the limited gatekeeping function

contemplated by Rule 702. In a 145-page fire origin and

ELOSU V. MIDDLEFORK RANCH 17

cause report applying the NFPA 921, Koster hypothesized

that the refrigerator pilot light ignited combustible vapors

produced by evaporated Penofin oil, producing a fast-

moving conflagration that swept along the deck and ignited

the stain on the southeast deck. Koster arrived at this

conclusion by conducting an analysis of the fire scene and

applying known scientific principles and established fire

investigation methodology. He studied the remains of the

cabin, examined evidence of fire movement and oxidation

patterns, and reviewed recorded interviews, deposition

transcripts, and video footage of the fire. He examined the

propane refrigerator and interviewed homeowners who were

present near the cabin in the days and hours leading up to the

fire. And he used before-and-after photographs to calculate

flame lengths and mass loss, referencing scientific literature

on the burning of milled lumber and volatile liquids.

The district judge did not question Koster’s

qualifications or the reliability of his methodology. Instead,

the district court solely took issue with “his ultimate

conclusions,” adopting Middlefork’s contention that they

were “speculative and unsupported by the evidence.” In its

analysis of Koster’s proposed testimony, the court found that

only limited portions of his report were substantive, that the

underlying facts were susceptible to competing

interpretations, that he relied too heavily on the Appellants’

story of events, and that his conclusions conflicted with the

testimony of the eyewitnesses. This analysis exceeds the

district court’s limited gatekeeping role in several important

respects.

First, the district court overlooked much of the scientific

analysis that formed the basis of Koster’s testimony. The

court concluded that only 20 to 25 pages of Koster’s 145-

page report were substantive in nature, distinguishing the

18 ELOSU V. MIDDLEFORK RANCH

remainder as “photographs, charts, and other supporting

documents.” But those “supporting documents” formed an

importing part of the “facts and data” that the district court

found lacking—including photographs and artifacts

obtained from the fire site, weather data, a forensics report,

and a reconstruction of the cabin. Accordingly, the same

error that demanded reversal in Kennedy is present here:

“Although the district court properly may exclude expert

testimony if the court concludes too great an analytical gap

exists between the existing data and the expert’s conclusion,

[this] gap was of the district court’s making.” 161 F.3d

at 1230. A court cannot exclude expert testimony for lacking

“sufficient facts or data” while openly disregarding the

foundation of the expert’s opinion.

Second, throughout its opinion, the district court

weighed the evidence and discredited Koster’s “ultimate

conclusions.” The court emphasized that Koster’s theory

was “directly contradicted by multiple eye-witness

accounts” of the fire, as the contractors had seen a fire raging

on the southeast deck, and Rauch had stepped onto the

northern deck to attempt to alert the occupants of the cabin.

The court took issue with Koster’s reliance on Brace’s

“interested” testimony, noting that Brace had hired Koster to

perform his analysis, and that Koster had made several

important assumptions “without conducting any

independent investigation.” And it weighed Koster’s report

against Johnson’s, observing that Middlefork’s expert had

“based his conclusions on other independently verifiable

facts and supporting evidence.” As Appellants note, this

analysis “fixat[ed] on evidence not offered in support of

[Koster’s] opinion while simultaneously ignoring the

evidence advanced on its behalf,” and exceeded the scope of

the Rule 702 inquiry. Cf. Daubert II, 43 F.3d at 1318 (“[T]he

ELOSU V. MIDDLEFORK RANCH 19

test under Daubert is not the correctness of the expert's

conclusions but the soundness of his methodology.”).

As an initial matter, these factual conclusions appear to

be clearly erroneous. See Lukashov, 694 F.3d at 1114.

Contrary to the district court’s contention that Koster did not

conduct an “independent investigation,” Koster’s report

details his exhaustive personal examination of the fire scene,

studying factors such as mass loss, wind flow, fire spread,

and material heat release rates. His theory was also fully

consistent with the eyewitness testimony. Although there

was no fire on the northern deck when the contractors

arrived, the court’s singular focus on this fact misses the

point of Koster’s hypothesis: his theory was that an open-

flame pilot light caused flammable vapors to ignite a

sustained fire on the southeast deck before the first

responders witnessed the fire. Cf. NFPA 921 § 19.3.1.5

(“Gases, vapors, and combustible dusts . . . can cause

confusion about the location of the point of origin, because

the point of ignition can be some distance away from where

sustained fire starts in the structure or furnishings.”).

Accordingly, it was clear error for the district court to

construe Koster’s testimony as incompatible with the

statements of witnesses on the scene. Whether the first

responders initially saw fire on the north deck is not

dispositive, as they arrived after the fire had begun and did

not witness the moment of ignition.

Given this logic, it is also notable that the court found

Johnson’s report admissible. Johnson theorized that the oil

on the deck had spontaneously combusted due to discarded

oil-soaked rags or an oil-soaked mop head. The district court

distinguished these theories from Koster’s by noting “that

Johnson based his conclusions on other independently

verifiable facts and supporting evidence.” But Middlefork

20 ELOSU V. MIDDLEFORK RANCH

acknowledges that none of the experts investigating the

scene could account for the oil-soaked rags, and Johnson

conceded there was no “direct evidence” of his theory.

Accordingly, Johnson’s testimony is no more “concrete”

than Koster’s. It was clear error to conclude otherwise, and

it was an abuse of discretion to select between these experts’

competing versions of events. Cf. City of Pomona, 750 F.3d

at 1049 (“Where two credible experts disagree, it is the job

of the fact finder, not the trial court, to determine which

source is more credible and reliable.”).

In any event, these concerns are matters for

impeachment, not admissibility. In performing its

gatekeeping function, a district court “is not tasked with

deciding whether [Koster] is right or wrong, just whether his

testimony has substance such that it would be helpful to a

jury.” Alaska Rent-A-Car, 738 F.3d at 969–70. NFPA 921

instructs fire investigators to rely on the observations of

witnesses and property owners when determining the origin

and cause of a fire. See NFPA 921 §§ 18.3.3.14, 19.3.1.6,

19.3.3. Koster acted in accordance with this professional

guideline when he relied on Brace’s opinion, which was

relevant to the central issue of the case: Brace applied the

Penofin oil stain to the deck and was one of the last people

to observe the deck before the cabin caught fire. It may well

be that Koster relied too heavily on an “interested party,”

that his report was not sufficiently corroborated, or that he

was biased towards Appellants, financially or otherwise. As

in Alaska Rent-A-Car, these countervailing concerns “go to

the weight of the testimony and its credibility, not its

admissibility.” 738 F.3d at 970; accord Primiano, 598 F.3d

at 564 (“Shaky but admissible evidence is to be attacked by

cross examination, contrary evidence, and attention to the

burden of proof, not exclusion.”).

ELOSU V. MIDDLEFORK RANCH 21

Finally, the court erred by demanding “concrete physical

or testimonial evidence” in a field characterized by

uncertainty. The court acknowledged that “Koster was able

to point to various factors that influenced his opinion,” but

found that “each of those factors have numerous alternative

explanations that could lead to alternative outcomes as

well.” By way of example, the court noted that the oxidation

Koster observed on the north side of the cabin was consistent

with the presence of strong north-to-south winds on the day

of the fire, that none of the witnesses he had interviewed

suggested that the point of ignition was on the north deck,

and that Koster lacked data supporting his theory that

Penofin oil had pooled beneath the deck on the following the

stain. The court concluded that the presence of “competing

interpretation[s]” and the lack of “concrete physical or

testimonial evidence” in support of Koster’s theory rendered

it too speculative to be admissible.

Quite simply, Koster is a fire investigator. The fact that

his testimony relied on circumstantial evidence and

inferences is neither unusual nor unexpected, as fires

routinely destroy all evidence of their origins. “By the very

nature of a fire, its cause must often be proven through a

combination of common sense, circumstantial evidence and

expert testimony.” Ricci v. Alt. Energy Inc., 211 F.3d 157,

162–63 (1st Cir. 2000) (quoting Minerals & Chems. Philipp

Corp. v. S.S. Nat’l Trader, 445 F.2d 831, 832 (2d Cir.

1971)); Hartley v. St. Paul Fire & Marine Ins. Co., 118 F.

App’x 914, 919 (6th Cir. 2004) (quoting same).

Accordingly, fire investigation, no less than medicine,

requires sound judgment in the face of uncertainty. Cf.

Primiano, 598 F.3d at 566. An expert in either field must

“use their knowledge and experience as a basis for weighing

known factors along with the inevitable uncertainties.” Id.;

Joiner, 522 U.S. at 146 (noting that experts regularly

22 ELOSU V. MIDDLEFORK RANCH

“extrapolate from existing data”); Daubert, 509 U.S. at 592

(emphasizing that an expert need not testify based on

“firsthand knowledge or observation”). The court’s

responsibility is to ensure that a sufficiently qualified expert

applied reliable principles to form their hypothesis—not to

gauge whether that hypothesis is ultimately correct. That is

for the litigants to prove, and for the factfinder to decide.

It is undisputed that Koster was qualified as an expert in

fire investigation, and that he applied broadly accepted

scientific principles and professional standards to conduct

his analysis. Koster is a Fire and Arson Investigator

employed by Reliant Investigations, Inc., with multiple

certifications and twenty-two years of experience in the

field. He reached his conclusions by applying NFPA 921,

which has been consistently accepted as a suitable

foundation for fire investigation testimony. See Schlesinger

v. United States, 898 F. Supp. 2d 489, 504 (E.D.N.Y. 2012)

(collecting cases); McCoy v. Whirlpool Corp., 214 F.R.D.

646, 653 (D. Kan. 2003) (calling NFPA 921 “gold standard”

for fire investigations). Those guidelines provide direct

support for Koster’s vapor fire theory. See, e.g., NFPA 921

§ 19.1.3 (noting that vapor fires often ignite far from the

location of the first sustained heavy fire and leave “no

physical evidence of an ignition source” at the origin of the

fire.). They also instruct fire investigators to rely on the

precise kinds of circumstantial evidence that formed the

basis of Koster’s opinion. See, e.g., NFPA 921 § 18.1.2

(directing investigators to examine witness statements, fire

patterns, arc mapping, and fire dynamics). Accordingly, “the

gate could not be closed to this relevant opinion offered with

sufficient foundation by one qualified to give it.” Primiano,

598 F.3d at 568.

ELOSU V. MIDDLEFORK RANCH 23

CONCLUSION

We REVERSE the district court’s partial grant of

Middlefork’s motion to exclude expert testimony and the

subsequent consent entry of summary judgment, and

REMAND this case to the United States District Court for

the District of Idaho for further proceedings consistent with

this opinion.