# Brown v. Southeastern Pennsylvania Transportation Authority

> Court of Appeals for the Third Circuit · August 31, 1994 · 35 F.3d 717

URL: https://www.frixlaw.com/law-library/cases/6932215

## Case

- **Full name:** In re PAOLI RAILROAD YARD PCB LITIGATION. Mabel BROWN, Individually and on behalf of all others similarly situated v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America Roy F. Weston, Inc. and OH Materials Company and General Electric Company and the Budd Company and Westinghouse Electric Corporation Monsanto Co. Penn Central Corporation, Mabel Brown, (D.C.Civ. No. 86-02229). In re PAOLI RAILROAD YARD PCB LITIGATION. George Albert BURRELL and Priscilla Etheridge Burrell, in their, own right, and George Albert Burrell and Priscilla Etheridge Burrell, as parents and natural guardian of Amber Shardai Burrell, a minor, and George Albert Burrell, as parent and natural guardian of Andre Walker, a minor, and Priscilla Etheridge Burrell, as parent and natural guardian of Bobby George Albert Christian Burrell, a minor v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") and National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America Monsanto Company General Electric Company the Budd Company Westinghouse Electric Corporation v. PENN CENTRAL CORPORATION, George Burrell, Priscilla Burrell, Amber Burrell and Monica Hilton, (D.C.Civ. No. 86-02235). In re PAOLI RAILROAD YARD PCB LITIGATION. Wallace Darryl CUMMINS v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") and National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation, ("Conrail") v. UNITED STATES of America and General Electric Company The Budd Company Westinghouse Electric Corporation Monsanto Company, Wallace D. Cummins, (D.C.Civ. No. 86-02669). In re PAOLI RAILROAD YARD PCB LITIGATION. K. Louise JONES, Administratrix of the Estate of Harvey N. Jones, Jr., and K. Louise Jones as Personal Representative of Harvey N. Jones, Jr., and K. Louise Jones in her own Right v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") National Railroad Passenger Corporation ("Amtrak") Consolidated Rail Corporation, ("Conrail") v. UNITED STATES of America City of Philadelphia Monsanto Company General Electric Company the Budd Company and Westinghouse Electric Corporation, K. Louise Jones and Harvey Jones, (D.C.Civ. No. 86-05277). In re PAOLI RAILROAD YARD PCB LITIGATION. James LAMENT, individually and on behalf of all others similarly situated v. SEPTA Amtrak and Conrail v. PENN CENTRAL CORPORATION United States of America City of Philadelphia Monsanto Co. General Electric Co. Budd Co., and Westinghouse Electric Corporation, James Lament, (D.C.Civ. No. 86-05886). In re PAOLI RAILROAD YARD PCB LITIGATION. Christopher S. BROWN Jacqueline M. Brown, h/w v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation Consolidated Rail Corporation ("Conrail") v. PENN CENTRAL CORPORATION United States of America City of Philadelphia General Electric Company The Budd Company Westinghouse Electric Corporation, Christopher Brown and Jacqueline Michelle Brown, (D.C.Civ. No. 86-07414). In re PAOLI RAILROAD YARD PCB LITIGATION. Cathlene BROWN v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America City of Philadelphia General Electric Company the Budd Company Westinghouse Electric Corporation v. PENN CENTRAL CORPORATION (D.C.Civ. No. 86-07415). In re PAOLI RAILROAD YARD PCB LITIGATION. Craig A. BROWN and Catherine D. Brown, h/w v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America Penn Central Corporation and City of Philadelphia General Electric Co. the Budd Co. and Westinghouse Electric Corp., Craig Brown, (D.C.Civ. No. 86-07416). In re PAOLI RAILROAD YARD PCB LITIGATION. Margherita BARBETTA v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America and City of Philadelphia the General Electric Company and the Budd Company and Westinghouse Electric Corporation PENN CENTRAL CORPORATION (D.C.Civ. No. 86-07417). In re PAOLI RAILROAD YARD PCB LITIGATION. Mary Retta JOHNSON v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America and the City of Philadelphia and General Electric Company the Budd Company and Westinghouse Electric Corporation and Penn Central Corp. (D.C.Civ. No. 86-07418). In re PAOLI RAILROAD YARD PCB LITIGATION. Celeste BROWN v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America City of Philadelphia and General Electric Company the Budd Company Westinghouse Electric Corporation (D.C.Civ. No. 86-07419). In re PAOLI RAILROAD YARD PCB LITIGATION. Clemmon L. BROWN v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America City of Philadelphia and General Electric Company the Budd Company and Westinghouse Electric Corporation and Penn Central Corporation, Clemmon Brown, (D.C.Civ. No. 86-07420). In re PAOLI RAILROAD YARD PCB LITIGATION. Cloyd H. BROWN v. MONSANTO COMPANY Southeastern Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America City of Philadelphia General Electric Company the Budd Company Westinghouse Electric Corporation Penn Central Corporation, Cloyd Brown, (D.C.Civ. No. 86-07421). In re PAOLI RAILROAD YARD PCB LITIGATION. Curtis BROWN v. MONSANTO COMPANY Southeastern , Pennsylvania Transportation Authority ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America City of Philadelphia v. GENERAL ELECTRIC COMPANY and the Budd Company and Westinghouse Electric Company Penn Central Corp. (D.C.Civ. No. 86-07422). In re PAOLI RAILROAD YARD PCB LITIGATION. John INGRAM, Sr., and Patricia Ingram, in their own right and as parents and natural guardians of John Ingram, Jr. and April Ingram in her own right v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") and Monsanto Company ("Monsanto") and General Electric Company ("GE") and City of Philadelphia ("Philadelphia") v. UNITED STATES of America the Budd Company Westinghouse Electric Corporation Penn Central Corporation, John Ingram, Sr. (and as representative of decedent, Patricia Ingram), John Ingram, Jr., Patricia Ingram and April Ingram Robinson-Ray, (D.C.Civ. No. 86-07561). In re PAOLI RAILROAD YARD PCB LITIGATION. Mary Alice KNIGHT v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") and Monsanto Company ("Monsanto") and General Electric Company ("GE") and City of Philadelphia ("Philadelphia") v. UNITED STATES of America the Budd Company Westinghouse Electric Corporation Penn Central Corporation (D.C.Civ. No. 87-00712). In re PAOLI RAILROAD YARD PCB LITIGATION. William BUTLER Theresa Butler Marvin L. Simpson Allen K. Simpson Karen R. Simpson Donald E. Simpson and Bryan M. Jackson v. SOUTHEASTERN PENNSYLVANIA TRANSPORTATION AUTHORITY ("SEPTA") National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. UNITED STATES of America City of Philadelphia Monsanto Company General Electric Company The Budd Company Westinghouse Electric Corporation Penn Central Corporation, William Butler, Theresa Butler, Bryan Jackson, Allen Simpson, Donald Simpson, Karen Simpson and Marvin Simpson, (D.C. Civ. No. 87-02874). In re PAOLI RAILROAD YARD PCB LITIGATION. Matthew CUNNINGHAM and Bessie Cunningham v. MONSANTO COMPANY and Southeastern Pennsylvania Transportation Authority ("SEPTA") and National Railroad Passenger Corporation ("Amtrak") and Consolidated Rail Corporation ("Conrail") v. GENERAL ELECTRIC COMPANY the Budd Company Westinghouse Electric Corporation Penn Central Corporation, Third-Party (D.C. Civ. No. 87-05269)
- **Court:** Court of Appeals for the Third Circuit
- **Decided:** August 31, 1994
- **Citations:** 35 F.3d 717
- **Precedential status:** Published
- **Opinion:** Opinion of the court by Becker
- **Judges:** Becker, Lewis, Roth
- **Cited by:** 9 later opinions in the Frix Law Library

## Citator (automated)

- No negative treatment found by the automated citator. That is not the same as a confirmation that the case is good law; read the citing cases.
- Full citator and citing cases: https://www.frixlaw.com/law-library/cases/6932215

## Opinion text

TABLE OF CONTENTS
I. INTRODUCTION 782
II. FACTS AND PROCEDURAL HISTORY 734
III. SUBJECT MATTER JURISDICTION OF THE DISTRICT COURT 737
IV. WAS THE IN LIMINE HEARING TAINTED BY UNFAIR PROCEDURES? 738
A. Inadequate Opportunity to Depose Defense Experts 738
B. Exclusion of Opinion of Experts for Whom No. 26(b)(4) Statements Were Filed 739
V. LEGAL STANDARDS FOR THE ADMISSIBILITY OF EXPERT OPINION 741
A. Rule 702 741
1. Qualifications 741
2. Reliability 742
3. Fit 742
4. The roles of judge and jury: how high a threshold for reliability? 743
5. Confusion — Rules 702 and 403 746
6. Procedural concerns regarding the Rule 403/702 balancing test 747
B. Rule 703 747
C. Standard of Review 749
D. The Significance of Pennsylvania’s Requirement of Reasonable Medical Certainty 750
VI. THE RELATIONSHIP BETWEEN PCBs AND PLAINTIFFS’ ILLNESSES 752
A. Dr. Sherman’s Qualifications 753
B. Rule 703 and Dr. Sherman’s Immunological Tests 754
C. Adequacy of Dr. Sherman’s and Dr. DiGregorio’s Use of Differential Diagnosis in Evaluating Causation of Present Illnesses 755
1. The district court’s critique 755
2. The plaintiffs’ response 757
3. Rule 702 analysis 758
a. The opinions of Dr. DiGregorio 763
b. The opinions of Dr. Sherman 764
i. Bessie Cunningham 765
ii. Amber Burrell 766
iii. Priscilla Burrell 767
iv. Monica Hilton 768
v. Matthew Cunningham 768
vi. Patricia Ingram 769
vii. John Ingram, Sr. 769
viii. John Ingram, Jr. 769
ix. George Burrell 769
x. Wallace Cummins 769
xi. April Ingram 770
xii. The Remaining Plaintiffs 770
c. Rule 403 770
d. Summary (DiGregorio and Sherman) 770
VII. EXPOSURE — THE OPINION OF IAN C.T. NISBET, Ph.D. 771
A. Recalculation of the AML Data 772
B. Recalculation of Background 773
C. Back Calculations 774
D. Remaining Issues 778
E. Conclusion 778
VIII. THE HARMFULNESS OF THE CHEMICALS 778
A. Introduction 778
B. Animal Studies 779
*732 C. Exposure to Dioxins and Furans 781
IX. THE YUSHO AND YU CHENG INCIDENTS 783
X.THE PLAINTIFFS’ PERSONAL INJURY CLAIMS — SUMMARY AND CONCLUSION 784
XI. THE MEDICAL MONITORING CLAIMS 785
A. Medical Monitoring as a Viable Claim 785
B. The Medical Monitoring Test Restated 787
C. Application of the Test 788
1. Admissibility of plaintiffs’ evidence 789
a. Dr. Sherman’s testimony 789
b. Dr. DiGregorio’s testimony 790
i. Reliability 790
ii. Failure to comply with the scheduling order 791
c. Dr. Sehecter’s testimony 1 793
2. Summary judgment on medical monitoring 793
XII. PROPERTY DAMAGE 795
XIII. CONCLUSION 798
Before BECKER, ROTH, and LEWIS, Circuit Judge.
OPINION OF THE COURT
BECKER, Circuit Judge.
I. INTRODUCTION
The plaintiffs in this toxic tort case have lived for many years in the vicinity of the Paoli Railyard, a railcar maintenance facility at which polychlorinated biphenyls (PCBs) were used in profusion for over a quarter century. They have sued to recover damages for a variety of physical ailments and for property damage against the corporations that have maintained the railyard and that sold the PCBs.
This appeal is from the district court’s second grant of summary judgment for the defendants. In In re Paoli R.R. Yard PCB Litig., 916 F.2d 829 (3d Cir.1990) (“Paoli I”), cert. denied, 499 U.S. 961 , 111 S.Ct. 1584 , 113 L.Ed.2d 649 (1991), we reversed the grant of summary judgment for defendants and remanded for further proceedings. We did so because the foundation of the summary judgment — the exclusion of virtually all the plaintiffs’ expert opinion pursuant to Federal Rules of Evidence 702, 703 and 403 — was undermined by (1) the failure of the district court to permit sufficient development of the record upon which a determination to exclude expert evidence might be based, and (2) the district court’s failure to make findings, grounded on Fed.R.Evid. 702 or 703, setting forth the basis of its decision.
On remand the district court conducted five days of in limine hearings, receiving extensive evidence about the scientific reliability of plaintiffs’ expert opinions. It then filed extensive opinions (totalling 330 pages) setting forth not only findings of fact but also its reasons for again excluding the vast bulk of plaintiffs’ expert evidence. This appeal followed.
Resolution of the appeal requires that we address myriad issues of procedure, evidence, and substantive law. Primarily, however, we must consider the voluminous record concerning expert opinion and, applying Fed.R.Evid. 702 and the standards enunciated by the Supreme Court in Daubert v. Merrell Dow Pharm., Inc., — U.S. —, 113 S.Ct. 2786 , 125 L.Ed.2d 469 (1993), decide whether the district court erred in again excluding the opinions of plaintiffs’ experts in connection with its summary judgment determination. Daubert requires the district court to act as “gatekeeper” and to assure that the scientific methodology upon which the expert opinion is founded is reliable, i.e., that the expert’s conclusion is based on good grounds (the methods and principles of science). We also must deal with Fed.R.Evid. 703 because the district court’s exclusion of expert testimony rests, in part, on its conclusion that some of the hearsay evidence upon which the experts relied is not of a kind usually relied on by experts in the field. In this regard.we conclude that the precept announced in In re: Japanese Electronics Prod. Antitrust Litig., 723 F.2d 238 (3d Cir. *733 1983), rev’d on other grounds sub nom., Matsushita Electric. Indus. Co. v. Zenith Radio Corp., 475 U.S. 574 , 106 S.Ct. 1348 , 89 L.Ed.2d 538 (1986), that the principal arbiters of the reasonableness of reliance upon inadmissible evidence are the experts and not the trial judge, does not survive Daubert .
Preliminary to an application of Rules 702-703 to the evidentiary rulings at issue, we must address the question of the proper scope of review. Although review of the district court’s fact findings that undergird its rulings on the admissibility of expert opinion is deferential, given the enormous power of the district court to foreclose submission of a party’s case to a jury on the basis of a threshold determination of nonreliability of opinion evidence, we conclude that the review requires a “hard look” to insure that the district court’s exercise of discretion was sound and that it correctly applied the several Daubert factors. Then to the extent that any of the expert opinion survives, we must determine whether the plaintiffs can avoid summary judgment on any of the claims to which the surviving opinion pertains.
For the most part we will affirm the rulings and the judgment of the district court. However, we also will reverse in part, and remand some claims for trial. In summary we rule as follows.
1. We will affirm the district court’s decision to retain jurisdiction in this case based on pendent jurisdiction stemming from the plaintiffs’ prior CERCLA claims.
2. We will affirm the district court’s discretionary ruling allowing certain of defendants’ experts to testify at the in limine hearings without being deposed; however, we point out that depositions should generally be afforded in future cases of this nature. We do not reach the propriety of its decision excluding plaintiffs’ experts for whom no Rule 26(b)(4) statements were filed because plaintiffs do not rely on these experts in opposition to summary judgment.
3. We will reverse the district court’s exclusion of the opinion of Dr. Janette D. Sherman insofar as it was grounded on the conclusion that she was an unqualified expert. However, we affirm the district court’s holding that she could not rely on immunological tests from an unaccredited laboratory. We also affirm the district court’s exclusion of the opinions of Dr. Sherman as to causation of plaintiffs’ present injuries with respect to those plaintiffs whom she did not examine and whose medical history she did not take. We believe that, in the absence of employment of the standard techniques of differential diagnosis, and in the absence of any good explanation justifying her conclusion in the light of possible alternative causes of plaintiffs’ injuries, Dr. Sherman’s methodology as to those plaintiffs was not reliable, as the district court found.
4. We will affirm the district court’s exclusion of the opinion of Dr. G. John DiGre-gorio with respect to causation of the injuries of those plaintiffs for whom he testified. Because Dr. DiGregorio neither examined these plaintiffs nor took their medical histories, he did not have a reliable basis from which to conclude they were ill. Moreover, as with Dr. Sherman, in the absence of employment of standard techniques of differential diagnosis, and failure to provide any explanation as to why he concluded that PCBs rather than alternative possibilities had caused plaintiffs’ illnesses, we conclude that Dr. DiGregorio’s assessment of causation was unreliable.
5. We will reverse the district court’s exclusion from evidence of the opinions of Dr. Sherman with respect to most of the injuries of Bessie Cunningham and Amber Burrell, whom she actually examined and whose medical histories she took. In light of: (1) Dr. Sherman’s employment of the standard techniques of differential diagnosis with respect to those plaintiffs, and (2) defendants’ failure to point to possible alternative causes of their injuries which Dr. Sherman failed to consider, we are satisfied that, with respect to them, Dr. Sherman’s scientific methodology was reliable and that the district court’s exclusion of her opinion represented an impermissible jury-like determination that it preferred the defendants’ experts’ opinion to that of Dr. Sherman.
6. We will affirm the district court’s ex-elusion from evidence of the opinion of Dr. Ian Nisbet regarding plaintiffs’ exposure to PCBs: (1) insofar as that opinion is based on *734 a recalculation of American Medical Laboratory test (“AML”) results to account for the degree to which the laboratory ostensibly underestimated its results; and (2) insofar as it is grounded on “back calculations” of plaintiffs’ PCB blood levels in 1986 based upon the results of studies made by the Eco Logic laboratory in 1992. We conclude that Dr. Nisbet’s methodology in recalculating the AML tests was unreliable, and that the Eco Logic data from which he began his back calculations was not a type reasonably relied on by experts in the field due to the quality control problems in Eco Logic’s methodology-
7. We will reverse the district court’s exclusion of the remainder of Dr. Nisbet’s opinion, including his conclusion that plaintiffs had PCBs in their blood at levels above the background PCB levels in the general population. We are satisfied that Dr. Nisbet’s calculation of background blood levels from National Human Adipose Tissue Survey (“NHATS”) fat data was based on a reliable methodology, and that the NHATS data was a type of data reasonably relied on by experts.
8. We will reverse the district court’s exclusion of animal studies. Where, as here, these studies were supported by some epidemiological data and had been used by EPA to conclude that PCBs were a probable human carcinogen, plaintiffs’ experts had good grounds for determining that they “fit” a conclusion regarding human causation.
9. We will affirm the district court’s exclusion of data on chemicals other than PCBs and its exclusion of testimony related to the incidents at Yusho (in Japan) and Yu Cheng (in Taiwan). The district court’s conclusion under Fed.R.Evid. 403 that the probative value of this evidence would have been substantially outweighed by unfair prejudice (and would have resulted in significant waste of trial time for resolution of collateral issues) was based on a more than adequate record (one justifying the rare making of the pretrial ruling based on Rule 403), and on findings of fact which, under the applicable standard of review we cannot disturb.
10. We will reverse the grant of summary judgment with respect to those personal injury plaintiffs for whom Dr. Sherman’s testimony is admissible, concluding that they have demonstrated the existence of a genuine issue of material fact as to whether defendants exposed them to PCBs which caused their present injuries.
11. We will reverse the grant of summary judgment with respect to the medical monitoring claims. Although recent changes in Pennsylvania law have interred plaintiffs’ increased risk claims, we still believe that the Pennsylvania Supreme Court would recognize the plaintiffs’ medical monitoring claims, and are satisfied that Dr. Sherman’s testimony as to the plaintiffs’ future risk is sufficient to get them over the summary judgment threshold.
12. We will reverse the grant of summary judgment on plaintiffs’ claim with respect to diminution in property value. Based on an extrapolation from existing Pennsylvania case law, we predict that where (1) the defendants have caused some physical harm to plaintiffs’ property; (2) repair of this damage will not restore the value of the property to its prior level; and (3) there is at least some ongoing risk to the plaintiffs’ land, Pennsylvania would allow plaintiffs to recover damages for the diminution of their property value. On the present record, there remains a genuine issue of material fact on the permanency of damage issue.
13. With respect to all other claims, which represent the bulk of plaintiffs’ ease, the exclusion of the expert opinion leaves the plaintiffs with inadequate evidence to survive summary judgment. We will therefore affirm the grant of summary judgment on these remaining claims.
II. FACTS AND PROCEDURAL HISTORY
Beginning in the 1950s, if not before, PCBs were used at the Paoli Railroad Yard (the ‘Yard”) as a fire-resistant insulating fluid in railroad car transformers. PCBs gradually accumulated in the Yard and leaked off the Yard into groundwater and ultimately into the soil of nearby residences. (A1122-1123). Several studies from 1979-1986 documented high levels of PCBs in the Yard and in *735 nearby water and land. The National Institute for Occupational Safety and Health (NIOSH) identifies the site as containing the worst PCB contamination it has ever encountered during a health inspection.
In 1986, the United States sued SEPTA, Conrail and Amtrak under the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), 42 U.S.C. § 9601 , to compel the cleanup of the Yard. After the entry of five consent decrees, which primarily required the control of leakage from the Yard, EPA adopted a final plan, the Record of Decision, in July of 1992. This plan requires extensive excavation and treatment of soils both at the Yard and on nearby, residential property and streams, and erosion controls at the Yard.
The plaintiffs in this action consist of individuals who lived near the Yard in areas identified by EPA and by the railroad defendants’ contractor, Groundwater Technologies, Inc., as having experienced the most severe run-off from the Yard. The plaintiffs have adduced evidence of significant levels of PCBs in the soil at their homes. Many of the plaintiffs played in the soil at their homes while growing up, gardened in it, and ate vegetables grown from it. Many also regularly traversed the Yard on foot as a' short cut to their destinations.
In 1986, thirty eight plaintiffs brought suit in the District Court for the Eastern District of Pennsylvania. Some sought recovery for present injuries, (allegedly caused by exposure to PCBs and other assorted chemicals from the Yard, including polychlorinated dibenzo furans (“furans”) and polychlorinated dibenzo-p-dioxins (“dioxins”)). Some plaintiffs, many of whom have not yet suffered any physical injury, brought claims for emotional distress caused by fear of future injury, and for medical monitoring to decrease the likelihood of the future development of serious diseases. Finally, some of the plaintiffs have brought claims for the decrease in value to their property caused by the presence of PCBs on the land.
The defendants (in some or all of the cases) are Monsanto Corporation, the nation’s leading manufacturer of PCBs; General Electric Company, manufacturer of some of the railroad car transformers in which the PCBs were used; Amtrak, owner of the Yard since 1976; Conrail, which operated the facility between 1976 and 1983; the Southeastern Pennsylvania Transit Authority (SEPTA), which has operated the facility since 1983; and the City of Philadelphia, which is linked with SEPTA. SEPTA filed third-party complaints against Westinghouse Electric Corporation, manufacturer of some of the transformers used at the Yard, and the Budd Company, manufacturer of some of the railroad ears. Conrail filed a third part complaint against the Penn Central Corporation.
In November of 1988 the district court granted defendants’ motion for summary judgment against all plaintiffs on the personal injury claims because it held that all of plaintiffs’ evidence pertaining to significant exposure and causation, necessary elements of their claims, was inadmissible. The court excluded the evidence under Federal Rule of Evidence 702, which requires expert witnesses to be qualified and to use reliable scientific techniques, Rule 703 which requires experts to rely on data of a type reasonably relied upon by experts in the field, and Rule 403 which requires the probative value of evidence not to be substantially outweighed by unfair prejudice or undue delay. See In re Paoli R.R. Yard PCB Litig., 706 F.Supp. 358 (E.D.Pa.1988).
In Paoli I, 916 F.2d at 854 , this Court reversed the Rule 703 holdings because the district court had not created a sufficiently detailed factual record on which to base them and had not given “the plaintiffs sufficient opportunity to explore the issues upon which they were ultimately denied relief.” We reversed the district court’s Rule 702 holdings because it had construed the qualification requirement of Rule 702 too strictly, see id. at 855-56 , and because it had not created a detailed enough factual record to justify exclusion of the experts’ opinion under the reliability prong of Rule 702. See id. at 858-59 . Finally, we reversed the district court’s Rule 403 determinations, holding that Rule 403 exclusions should not be granted pretrial absent a record which is “a virtual surrogate for a trial record.” Id. at 859-60 . We strongly suggested that the district court *736 should have held an in limine hearing before excluding plaintiffs’ expert opinion. Id. at 859 .
Following remand, the district court issued a cáse management order in June 1991, giving plaintiffs until March 2, 1992 to complete their discovery, and until March 17, 1992 to designate their trial experts and provide defendants with their experts’ reports. The court required the defendants to do likewise one month after plaintiffs.
The list of expert witnesses designated by the plaintiffs in March of 1992 was considerably different from the list at the time of the 1988 summary judgment. The twenty-three plaintiffs represented by the Kohn, Nast, & Graf, and Klehr, Harrison, Harvey, Branz-burg, Ellers, & Weir law firms (the “Kohn/Klehr” plaintiffs) named three experts supporting their personal injury claims: Mel-vyn Kopstein, Ph.D., to testify about the Paoli residents’ opportunities for exposure to PCBs from the Yard; Ian C.T. Nisbet, Ph. D., to testify about the plaintiffs’ exposure to PCBs; and Janette Sherman, M.D., to testify that PCBs had caused plaintiffs’ injuries and that plaintiffs require medical monitoring to detect and treat future PCB-related illnesses.
Defendants moved in limine to exclude the opinions of each of these experts under Fed.R.Evid. 702, 703 and 403. On the same grounds, defendants filed motions in limine to preclude the plaintiffs’ experts from testifying about animal studies purporting to show the harmful effects of PCBs, evidence concerning the harm of dioxins and furans (chemicals that are sometimes present in transformer fluids), and evidence concerning the ‘Yusho” incident in Japan and the ‘Yu Cheng” incident in Taiwan in which many individuals suffered adverse effects after consuming rice oil contaminated with PCBs and furans.
Bruce Hanes, who represented the nine remaining plaintiffs (the “Hanes plaintiffs”), designated Dr. G. John DiGregorio, M.D. to testify that PCBs had caused their injuries, and Arnold Schecter, M.D., M.P.H., to testify that their fear of future illness was reasonable. In a joint pretrial memorandum filed on July 17, 1992, the Kohn/Klehr and Hanes plaintiffs designated three additional trial experts — Deborah Barsotti, Ph.D., Benjamin Calesnick, M.D., and Arthur Zahalsky, Ph. D. — who had offered opinions for the plaintiffs in 1988. They also listed William J. Nicholson, Ph.D. and Cate Jenkins, Ph.D. as fact witnesses. Defendants moved to exclude these witnesses because plaintiffs had failed to designate them as experts in March, as the district court’s pre-trial order had required.
The district court held five days of in limine hearings between July 28 and August 7,1992. Drs. Kopstein, Nisbet, and Sherman testified for the plaintiffs. Ten physicians and scientists testified for the defense. On October 21, 1992, the, court entered orders excluding all but one of plaintiffs’ experts (Dr. Kopstein). The court also excluded evidence concerning animal studies, dioxins and furans, and the incidents at Yusho and Yu Cheng. The court then granted summary judgment against the plaintiffs on their personal injury claims.
The court explained that under our holding in Paoli I, the plaintiffs had to establish the following elements to make out a prima facie case: (1) that the defendants had released PCBs into the environment; (2) that the plaintiffs had somehow ingested those PCBs into their bodies; (3) that the plaintiffs have an injury; and (4) that PCBs are the cause of their .injury. See Paoli I, 916 F.2d at 860 . Not surprisingly, after excluding plaintiffs’ experts, the court found that plaintiffs had no evidence on either exposure or causation and thus could not prove elements 2, 3, and 4. For similar reasons, the court granted summary judgment against the plaintiffs on their medical monitoring claims.
Nine plaintiffs — Mabel Brown, K. Louise Jones, James Lament, Christopher Brown, Margherita Barbetta, Mary Retta Johnson, John Ingram, William Butler, arid Matthew Cunningham — also brought property damage claims against the defendants. The district court granted summary judgment in favor of the defendants on these claims as well. The court held that Pennsylvania law only allows recovery for diminution of a property’s market value if there has been permanent damage to the property; and the court found *737 that plaintiffs could not prove such permanent damage in light of EPA’s proposed cleanup plan.
The plaintiffs challenge the district court’s subject matter jurisdiction, all of its admissibility decisions, its legal conclusions regarding diminution of a property’s market value, and the court’s grant of summary judgment. We will affirm most of the district court’s decisions. We will reverse only its grant of summary judgment on a number of the present injury claims of Bessie Cunningham and Amber Burrell, and its grant of summary judgment on the plaintiffs’ claims for medical monitoring and diminution in property value.
III. SUBJECT MATTER JURISDICTION OF THE DISTRICT COURT 1
Plaintiffs’ original claims in 1986 included a CERCLA claim, 42 U.S.C. § 9601 et seq., which conferred federal question jurisdiction, 28 U.S.C. § 1331 , on the district court. The court possessed pendent jurisdic-. tion over plaintiffs’ other claims. However, plaintiffs voluntarily dismissed the CERCLA claim after our ruling in Paoli I. Plaintiffs contend that, after this dismissal, the district court no longer had subject matter jurisdiction over the case because, absent the CERCLA claim, there was no federal question jurisdiction; nor was there complete diversity. 2
In United Mine Workers v. Gibbs, 383 U.S. 715 , 86 S.Ct. 1130 , 16 L.Ed.2d 218 (1966), the Supreme Court held that in evaluating whether to exercise pendent jurisdiction, a district court must weigh “considerations of judicial economy, convenience and fairness to [the] litigants.” 383 U.S. at 726 , 86 S.Ct. at U39. 3 In holding that these factors favored retention of jurisdiction here, the district court acted within the ambit of its discretion.
Considerations of judicial economy clearly weighed in favor of the district court retaining jurisdiction. Although considerations of judicial economy alone are generally insufficient to justify a district court’s decision to retain jurisdiction, see Shaffer v. Board of Sch. Directors , 730 F.2d 910 , 912 (3d Cir.1984); Lovell Mfg. v. Export-Import Bank of U.S., 843 F.2d 725 , 734-35 (3d Cir.1988), we have held such concerns sufficient when they are especially strong. In Lentino v. Fringe Employee Plans, Inc., 611 F.2d 474, 480 (3d Cir.1979), we upheld a district court’s decision to retain jurisdiction over a state law claim when plaintiffs dropped their federal claims on the morning of trial after there had already been a year of pretrial proceedings.
The concerns of judicial economy here are at least as strong as they were in Lentino . Although the plaintiffs dropped their CERC-LA claim much earlier than the morning before trial, far more energy had been invested in the proceedings here than in Lenti-no. Six years of federal proceedings had occurred before the plaintiffs dropped their CÉRCLA claim, including extensive discovery. Moreover, the concern over judicial economy was combined here with two other *738 factors strongly militating in favor of retention of jurisdiction.
First, the district court would have continued to have jurisdiction over plaintiffs’ claims against Amtrak, see 28 U.S.C. § 1349 (providing district court jurisdiction over all corporations incorporated by an act of Congress in which the United States owns more than half the stock) even if it had dismissed most of plaintiffs’ claims for lack of jurisdiction. Amtrak doubtless would then have impleaded the other defendants, all of whom had deep pockets, thus changing the alignment of the parties but not the nature of the litigation. In such circumstances, a district court acts within its discretion in retaining jurisdiction. See, e.g., New Jersey Dept. of Env. Prot. v. Gloucester Env. Mgmt. Services, Inc., 719 F.Supp. 325, 337 (D.N.J.1989) (retaining jurisdiction in order to prevent “a litigation merry-go-round”).
Second, plaintiffs were the ones who initially filed their claims in federal court and obtained a 1986 ruling that the CERCLA claims created federal jurisdiction. Plaintiffs dismissed their CERCLA claims and argued against federal jurisdiction only after the district court had first granted summary judgment against them. Thus, plaintiffs’ dismissal of their CERCLA claims may well constitute an attempt to forum shop by divesting the district court of jurisdiction. In Carnegie-Mellon Univ. v. Cohill, 484 U.S. 343 , 108 S.Ct. 614 , 98 L.Ed.2d 720 (1988), the Supreme Court held that the possibility of forum shopping is one factor a district court should take into account in exercising its discretion with respect to pendent jurisdiction. See id. at 351, 356 , 357 & n. 12, 108 S.Ct. at 619 , 622-23 & n. 12.
Cohill , to be sure, was not about whether a district court should retain jurisdiction, but rather about what a district court should do after deciding not to retain jurisdiction, i.e. whether the court should dismiss the case, which might end it due to statute of limitations problems in state courts, or whether the court should remand the case to the state court from whence it had been removed. But Cohill did hold that one factor the district court should consider in deciding whether to remand the case to state courts is whether plaintiffs are attempting to forum shop. We hold that the same principle applies when the district court decides whether to retain jurisdiction in the first place.
In sum, fairness to defendants along with judicial economy and convenience justify the district court’s decision to retain jurisdiction here.
IV. WAS THE IN LIMINE HEARING TAINTED BY UNFAIR PROCEDURES?
Before addressing the central issues in the case (regarding the admissibility of the opinions of the plaintiffs’ experts under Rules 702, 703 and 403), we must dispose of some preliminary questions. The plaintiffs contend that the district court should not have relied on the testimony of the defendants’ experts in excluding the testimony of the plaintiffs’ experts, because the defendants failed to designate these experts in accord with the time schedule announced by the district court. We review for abuse of discretion, and find none with respect to this ruling.
Plaintiffs also submit that the district court should not have relied on its scheduling order to exclude the testimony of some of plaintiffs’ proposed experts. We do not have to reach the merits of this contention, because plaintiffs did not point to the testimony of these experts in opposition to summary judgment.
A. Inadequate Opportunity to Depose Defense Experts
On June 4, 1991, the district court issued an order requiring defendants to “designate all trial expert witnesses” by April 17, 1992 and giving the parties “until June 17, 1992 to depose expert witnesses.” (emphasis added). On June 25, 1992, well after the deadline the district court had set for identification of experts, defendants submitted affidavits from a new set of experts. Defendants intended to have these experts testify at the August in limine hearing but not at trial. Because discovery was closed, plaintiffs were unable to depose them.
*739 Plaintiffs moved to prevent these experts from testifying. The district court never ruled on this motion, allowed the experts to testify at the in limine hearing, and relied on their testimony in excluding much of plaintiffs’ evidence. Plaintiffs contend that this was error. On a purely formal level, we disagree for, as defendants correctly assert, the district court’s June 1991 order setting deadlines for the identification of experts was only directed at trial witnesses and said nothing about witnesses who were to testify only at the in limine hearing. But that conclusion does not settle the question. By coincidence, plaintiffs’ more significant claim of error is encapsulated in a recent law review article by Professor Berger, who argues that “[e]ourts should not permit the defendant to obtain a hearing on a motion in limine by relying on affidavits from experts unless their identity and reports have been supplied to the plaintiff in the course of discovery and the plaintiff had an opportunity to depose [them].” Margaret A. Berger, Procedural Paradigms for Applying the Daubert Test, 78 Minn.L.Rev. 1345, 1372 (1994).
The district court certainly had the power to provide that the experts who were to testify at the in limine hearing be subject to discovery. See Berger, supra, at 1372 n. 141. (“A court undoubtedly has power pursuant to Federal Rule of Civil Procedure 16 to require parties to provide each other with expert reports and an opportunity for the deposition of experts retained in connection with in limine motions.”). And we generally agree with Professor Berger that because under Daubert a judge at an in limine hearing must make findings of fact on the reliability of complicated scientific methodologies and this fact-finding can decide the case, it is important that each side have an opportunity to depose the other side’s experts in order to develop strong critiques and defenses of their expert’s methodologies. See Paoli I, 916 F.2d at 854 (“[T]he detailed factual record requirement, firmly entrenched in our jurisprudence, requires adequate process at the evidentiary stage, particularly when a summary judgment may flow from it.” (internal citations omitted)). Given the “liberal thrust” of the federal rules, see Daubert, — U.S. at -, 113 S.Ct. at 2794 (internal quotation omitted), it is particularly important that the side trying to defend the admissibility of evidence be given an adequate chance to do so. Moreover, fairness suggests that each side should have an equal opportunity to depose the other side’s experts. 4
In this ease, however, where we had yet to articulate our specific concern with depositions prior to the ruling of the district court, we conclude that the district court did not abuse its discretion in allowing experts, who had provided affidavits, to testify at the in limine hearing without being deposed. 5 Moreover, the defendants submitted the affidavits of their experts four weeks prior to the hearing, so that the plaintiffs had a significant opportunity to prepare for the testimony of defendants’ experts. They also had extensive leeway at the in limine hearing, where the conservation of the time of the jury was not implicated, to consult with their experts regarding any new arguments presented by defendants’ experts. Under these circumstances, whatever unfairness to the plaintiffs that resulted from their inability to depose defendants’ experts is not sufficient to warrant a reversal for depositions and a new in limine hearing.
B. Exclusion of Opinion of Experts for Whom No. 26(b) (U) Statements Were Filed
In the June 4, 1991, scheduling order setting deadlines for designation of defense ex *740 perts discussed above, the district court required plaintiffs to designate on or before March 17, 1992, “all trial expert witnesses and provide defendants with the substance of their trial testimony and the basis of their opinions at the time of designation in accordance with Rule 26(b)(4).” On July 17, 1992, four months after the deadline for designating experts and two weeks before the scheduled trial date, the plaintiffs filed a joint pretrial memorandum in which they named three expert witnesses, Drs. Calesnick, Bar-sotti, and Zahalsky, who had not been identified in plaintiffs’ March 1992 Rule 26(b)(4) statements. Plaintiffs also named Drs. Nicholson and Jenkins as fact witnesses even though defendants argue that they have no connection with this case and are really expert witnesses. The district court excluded all of these witnesses on the basis that plaintiffs had not named them by March 17,1992.
Plaintiffs assert that because Drs. Cales-niek, Barsotti, and Zahalsky provided opinions in 1988 before the first summary judgment decision, the plaintiffs had designated them as experts before March 17, 1992, as required by the district court’s scheduling order. Moreover, plaintiffs argue, even if they violated the pre-trial order, the district court abused its discretion in sanctioning them by excluding the witnesses under the factors we set forth in Meyers v. Pennypack Woods Home Ownership Ass’n, 559 F.2d 894 (3d Cir.1977). We do not reach the merits of the district court’s ruling.
Plaintiffs did not rely on these witnesses to oppose summary judgment. In fact, the plaintiffs did not even list these witnesses as trial witnesses until July 17, nine days after they had filed their opposition to summary judgment. As a result, our consideration of whether the district court was correct to grant summary judgment does not depend on whether the district court properly excluded these witnesses. “Where a party opposing a motion for summary judgment has the burden of persuasion, and the moving party has identified sufficient facts of record to demonstrate that no genuine issue of material fact remains, the nonmoving party is obliged to identify those facts of record which would contradict the facts identified by the movant.” Childers v. Joseph, 842 F.2d 689 (3d Cir.1988). See also Celotex Corp. v. Catrett, 477 U.S. 317, 322 , 106 S.Ct. 2548, 2552-53 , 91 L.Ed.2d 265 (1986). If the plaintiffs wished to rely on the opinions of Drs. Cales-nick Barsotti, Zahalsky, Nicholson and Jenkins to oppose summary judgment, they were required to identify the relevant facets of this testimony in their opposition to summary judgment.
Nonetheless, because we are reversing the grant of summary judgment with respect to some plaintiffs and some issues, whether these witnesses can testify at trial may still matter on remand. We could therefore decide whether the district court’s original decision excluding these witnesses was an abuse of discretion, the matter having been properly put before us. However, we decline to do so, preferring to allow the district court to reconsider its decision in light of the additional time that will be available to the defendants to depose the witnesses.
In excluding the witnesses originally, the district court partly relied on the proximity of the trial date to the revelation by plaintiffs of their intent to call these experts. And proximity is indeed one important factor. 6 *741 When the plaintiffs named these experts in July of 1992, two weeks before trial, giving defendants a significant time to depose them would have required delaying a long scheduled trial in a complex case that had already gone on for years. But after remand, the district court will easily b'e able to give the defendants significant time to depose the plaintiffs. In sum, in light of the changed circumstances that will be present upon remand, we choose not to determine whether the district court’s original decision constituted an abuse of discretion.
Y. LEGAL STANDARDS FOR THE ADMISSIBILITY OF EXPERT OPINION
The district court excluded plaintiffs’ remaining experts under Rules 702, 703, and 403. While the district court, the plaintiffs, and the defendants basically agree on the legal standards to be applied in analyzing admissibility, they also have some significant differences which repeatedly surfaced throughout the disputes over the admissibility of particular testimony. Accordingly, before analyzing the admissibility of the testimony of particular experts, we will first provide an overview of the legal standards under which we will evaluate that admissibility. Such an overview is also important in light of the Supreme Court’s recent decision in Dau-bert, which, although it largely adopted the Rule 702 test we announced in Downing, works certain changes in our jurisprudence on the admissibility of expert opinions.
A. Rule 702
Rule 702 provides that:
If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.
1. Qualifications
Rule 702 has two major requirements. The first is that a witness proffered to testify to specialized knowledge must be an expert. We have interpreted this requirement liberally. See Paoli I, 916 F.2d at 855 . We have held that a broad range of knowledge, skills, and training qualify an expert as such. See id. at 855 . In Paoli I we ruled that the district court had abused its discretion in excluding the opinions of Drs. Barsotti, Za-halsky, and Nisbet. We explained that exclusion was not the proper remedy “simply because the experts did not have the degree or training which the district court apparently thought would be most appropriate.” Id. at 856 .
Defendants submit that when this court held that Rule 702 mandates a policy of liberal admissibility, all that it was doing was eliminating any requirement that a candidate have specific formal qualifications such as a degree in a particular field. They assert that we were simply directing the trial court to focus on the “substance of a proffered expert’s background and its fit with respect to the issue at hand.” Thus, defendants suggest, when a trial court evaluates an expert’s substantive rather than formal qualifications, it does not have to use a liberal policy of admission.
But this is incorrect. Rule 702’s liberal policy of admissibility extends to the substantive as well as the formal qualification of experts. We have eschewed imposing overly rigorous requirements of expertise and have been satisfied with more generalized qualifications. See Hammond v. International Harvester Co., 691 F.2d 646, 652-53 (3d Cir.1982) (holding that an engineer, whose only qualifications were sales experience in the field of automotive and agricultural equipment and teaching high school automobile repair, nevertheless could testify in a products liability action involving tractors); Knight v. Otis Elevator Co., 596 F.2d 84, 87-88 (3d Cir.1979) (holding that an expert could testify that unguarded elevator buttons constituted a design defect despite expert’s lack of specific background in design and manufacture of elevators). As we explain below, however, the level of expertise may affect the reliability of the expert’s opinion.
*742 2. Reliability
The second requirement of Rule 702 is that the expert must testify to “scientific, technical or other specialized knowledge [that] will assist the trier of fact.” Fed.R.Evid. 702. The Supreme Court recently evaluated the meaning of this requirement in Daubert and held that Rule 702 does not incorporate the common law Frye rule in which expert testimony is admissible only insofar as it is based on a technique generally accepted in the scientific community. Instead, the Supreme Court agreed with this Court’s basic holding in Downing that an expert’s testimony is admissible so long as the process or technique the expert used in formulating the opinion is reliable. See Daubert, — U.S. at -, 113 S.Ct. at 2794-95 (“That the Frye test was displaced by the Rules of Evidence does not mean, however, that the Rules themselves place no limits on the admissibility of purportedly scientific evidence. Nor is the trial judge disabled from screening such evidence. To the contrary, under the Rules the trial judge must ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable.”); Downing 753 F.2d at 1237. Daubert explains that the language of Rule 702 requiring the expert to testify to scientific knowledge means that the expert’s opinion must be based on the “methods and procedures of science” rather than on “subjective belief or unsupported speculation”; the expert must have “good grounds” for his or her belief. Daubert , — U.S. at-•, 113 S.Ct. at 2795 . In sum, Daubert holds that an inquiry into the reliability of scientific evidence under Rule 702 requires a determination as to its scientific validity. Id. at-, 113 S.Ct. at 2795 n. 9.
Daubert suggests several factors that a district court should take into account in evaluating whether a particular scientific methodology is reliable (i.e. scientifically valid), including the testability of the expert’s hypothesis (“whether it can be (and has been) tested”), Daubert , — U.S. at -, 113 S.Ct. at 2796 , whether the methodology has been subjected to peer review and publication, the frequency by which the methodology leads to erroneous results, the existence and maintenance of standards controlling the technique’s operation, and whether the methodology has been generally accepted in the scientific community. 7 See Daubert, — U.S. at-, 113 S.Ct. at 2796-97 . We listed very similar factors a district court should consider in Downing. See Downing, 753 F.2d at 1238-39. Daubert lists two factors not listed in Downing — whether a method produces testable hypotheses and the existence of standards controlling the technique’s operation, and Daubert leaves off its list several factors listed in Downing — the degree to which the expert testifying is qualified, the relationship of a technique to “more established modes of scientific analysis,” and the “non-judicial uses to which the scientific technique are put.” Downing, 753 F.2d at 1238-39; see Daubert, — U.S. at- -, 113 S.Ct. at 2796-97 . But Daubert specifically refuses to disavow any the particular factors we listed in Downing. See Dau-bert, — U.S.-, 113 S.Ct. at 2797 n. 12. And Daubert , like Downing, indicates that the inquiry as to whether a particular scientific technique or method is reliable is a flexible one. See id. at-, 113 S.Ct. at 2796 , Downing, 753 F.2d at 1238-39. We now make clear that a district court should take into account all of the factors listed by either Daubert or Downing as well as any others that are relevant. 8
3. Fit
In addition to reliability, Rule 702 requires that the expert’s testimony must *743 assist the trier of fact. As we put it in Downing, admissibility depends in part on “the proffered connection between the scientific research or test result to be presented and particular disputed factual issues in the case.” Downing, 753 F.2d at 1237. See Daubert, — U.S.-, 113 S.Ct. at 2795-96 (explicitly adopting the “fit” requirement of Downing). For example, animal studies may be methodologically acceptable to show that chemical X increases the risk of cancer in animals, but they may not be methodologically acceptable to show that chemical X increases the risk of cancer in humans. Daubert explains that, “ ‘[flit’ is not always obvious, and scientific validity for one purpose is not necessarily scientific validity for other, unrelated purposes.” Id. at-, 113 S.Ct. at 2796 . Thus, even if an expert’s proposed testimony constitutes scientific knowledge, his or her testimony will be excluded if it is not scientific knowledge for purposes of the case. “Rule 702’s ‘helpfulness’ standard requires a valid scientific connection to the pertinent inquiry as a precondition to admissibility.” Id. (emphasis added). For example, in order for animal studies to be admissible to prove causation in humans, there must be good grounds to extrapolate from animals to humans, just as the methodology of the studies must constitute good grounds to reach conclusions about the animals themselves. Thus, the requirement of reliability, or “good grounds,” extends to each step in an expert’s analysis all the way through the step that connects the work of the expert to the particular case.
4. The roles of judge and jury: how high a threshold for reliability?
Plaintiffs’ basic position in their challenge to each of the district court’s admissibility decisions is that the district court improperly usurped the role of the jury by the manner in which it analyzed admissibility. Plaintiffs first argue that, so long as they have made a prima facie showing that an expert’s testimony is reliable, it is up to the jury to evaluate the expert’s testimony. They assert that for the judge to evaluate competing evidence on reliability before determining admissibility would force plaintiffs to prove their case twice — once to the judge in order to persuade him or her to admit the evidence and once to the jury to persuade it of liability.
But plaintiffs’ position that a party who wants to introduce expert testimony must make only a prima facie showing that a technique is reliable (e.g., the testimony of a single qualified expert) is specifically foreclosed by precedent. Daubert holds that admissibility under Rule 702 is governed by Rule 104(a), which requires the judge to conduct preliminary factfinding, to make “a preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid,” and thus enables the judge to exclude evidence presented in plaintiffs’ prima facie case. 9 Daubert , — U.S. at -, 113 S.Ct. at 2796 . Similarly, we held in Downing that “it is plain that the proponent must make more than a prima facie showing .■.. that a technique is reliable.” Downing, 753 F.2d at 1240 n. 21. We added that, in contrast to the view that the admission of scientific testimony is a matter of conditional relevancy governed by Rule 104(b), “novel scientific evidence carries with it concerns over trustworthiness and reliability akin to those raised by offers of hearsay evidence. When there is a serious question of reliability of evidence, it is appropriate for the court to exercise to some degree an evidentiary screening function.” Id. Cf. In re Japanese Elec. Prod., 723 F.2d 238, 276-77 (3d Cir.1983) (implying that Rule 104(a) requires *744 judges to make factual evaluations in making Rule 703 determinations). 10
This does not megn that plaintiffs have to prove their ease twice — they do not have to demonstrate to the judge by a preponderance of the evidence that the assessments of their experts are correct, they only have to demonstrate by a preponderance of evidence that their opinions are reliable. 11 As the Supreme Court has explained in describing the effect of the preponderance standard of Rule 104(a) generally, “[t]he inquiry made by a court ... is not whether the proponent of the evidence wins or loses his case on the merits, but whether the evidentiary Rules have been satisfied.” Bourjaily v. United States, 483 U.S. 171, 175 , 107 S.Ct. 2775, 2778 , 97 L.Ed.2d 144 (1987).
The evidentiary requirement of reliability is lower than the merits standard of correctness. Daubert states that a judge ■ should find an expert opinion reliable under Rule 702 if it is based on “good grounds,” i.e., if it is based on the methods and procedures of science. A judge will often think that an expert has good grounds to hold the opinion that he or she does even though the judge thinks that the opinion is incorrect. As Dau-bert indicates, “[t]he focus ... must be solely on principles and methodology, not on the conclusions that they generate.” Daubert , — U.S. at -, 113 S.Ct. at 2797 . The grounds for the expert’s opinion merely have to be good, they do not have to be perfect. The judge might think that there are good grounds for an expert’s conclusion even if the judge thinks that there are better grounds for some alternative conclusion, and even if the judge thinks that a scientist’s methodology has some flaws such that if they had been corrected, the scientist would have reached a different result.
As we explained in Paoli I, “the reliability requirement must not be used as a tool by which the court excludes all questionably reliable evidence.” Paoli I, 916 F.2d at 857 . The “ultimate touchstone is helpfulness to the trier of fact, and with regard to reliability, helpfulness turns on whether the expert’s ‘technique or principle [is] sufficiently reliable so that it will aid the jury in reaching accurate results.’ ” DeLuca, 911 F.2d at 956 (quoting 3 J. Weinstein & M. Berger, Weinstein’s Evidence 702[03], at 702-35 (1988)). 12 A judge frequently should find an expert’s *745 methodology helpful even when the judge thinks that the expert’s technique has flaws sufficient to render the conclusions inaccurate. He or she will often still believe that hearing the expert’s testimony and assessing its flaws was an important part of assessing what conclusion was correct and may certainly still believe that a jury attempting to reach an accurate result should consider the evidence. See Paoli I, 916 F.2d at 857 (helpfulness requires more than bare logical relevance, but there is a strong preference for admission) (citing Downing, 753 F.2d at 1235).
The same standard of reliability extends to the step in the expert’s analysis that “fits” his or her conclusions to the case at hand. Once again, we emphasize that the standard is not that high. For example, in Paoli I, we held that testimony that PCBs cause liver cancer “fit” the case even in the absence of plaintiffs who had liver cancer, because an expert’s affidavit suggested that increased risk of liver cancer was probative of increased risk of other forms of cancer. See Paoli I, 916 F.2d at 858 . Nonetheless, the standard is higher than bare relevance. 13
In addition to arguing that the district court usurped the role of the jury by requiring more than a prima facie showing of reliability, plaintiffs submit that the district court mistakenly applied the reliability inquiry to expert testimony that should not have been subject to that inquiry. Both Daubert and Downing require that expert testimony be based on the methods and procedures, the processes and techniques of science. In Pao-li I we suggested that so long as any deviation the expert made from a rehable method merely constituted a change in the application of that method, the expert’s testimony remained based on a rehable scientific method. We though that ferreting out misapplication fell within the province of the jury, and that only if the expert so altered a rehable methodology as to skew it would the rehabihty inquiry of Downing be apphcable to the altered methodology. See Paoli I, 916 F.2d at 858 . Plaintiffs assert that the district court apphed the Downing inquiry to methodologies that merely constituted shght alterations of rehable methodologies.
However, after Daubert , we no longer think that the distinction between a methodology and its apphcation is viable. To begin with, it is extremely elusive to attempt to ascertain which of an expert’s steps constitute parts of a “basic” methodology and which constitute changes from that methodology. If a laboratory consistently fails to use certain quality controls so that its results are rendered unreliable, attempting to ascertain whether the lack of quahty controls constitutes a failure of methodology or a failure of apphcation of methodology may be an exercise in metaphysics. Moreover, any misapplication of a methodology that is significant enough to render it unreliable is likely to also be significant enough to skew the methodology.
As suggested, Daubert inters any need for us to make such a distinction, for Daubert’s requirement that the expert testify to scientific knowledge—conclusions supported by good grounds for each step in the analysis— means that any step that renders the analysis unrehable under the Daubert factors renders the expert’s testimony inadmissible. This is true whether the step completely changes a reliable methodology or merely misapplies that methodology. 14
*746 Finally, plaintiffs contend that the district court usurped the role of the jury by making admissibility decisions based on its disagreement with the conclusions of plaintiffs’ experts. Plaintiffs are correct, of course, that Daubert requires the judge’s admissibility decision to focus not on the expert’s conclusions but on his or her principles and methodology. See Daubert, — U.S. at-, 113 S.Ct. at 2797 . But we think that this distinction has only limited practical import. When a judge disagrees with the conclusions of an expert, it will generally be because he or she thinks that there is a mistake at some step in the investigative or reasoning process of that expert. If the judge thinks that the conclusions of some other expert are correct, it will likely be because the judge thinks that the methodology and reasoning process of the other expert are superior to those of the first expert. This is especially true given that the expert’s view that a particular conclusion “fits” a particular case must itself constitute scientific knowledge — a challenge to “fit” is very close to a challenge to the expert’s ultimate conclusion about the particular case, and yet it is part of the judge’s admissibility calculus under Daubert, 15
Thus, as we explained above, we think that the primary limitation on the judge’s admissibility determinations is that the judge should not exclude evidence simply because he or she thinks that there is a flaw in the expert’s investigative process which renders the expert’s conclusions incorrect. The judge should only exclude the evidence if the flaw is large enough that the expert lacks “good grounds” for his or her conclusions.
5. Confusion — Rule 403
In Downing we explained that under Rule 702, admissibility of scientific testimony turns not only on reliability but also on the possibility that admitting the evidence would overwhelm, confuse, or mislead the jury. See Downing, 753 F.2d at 1240. We held that in conducting this balancing inquiry, there is a presumption of helpfulness. See id. at 1241 (citing Japanese Elec. 723 F.2d at 279 ). The extent to which an adverse party has had notice and the opportunity to present his or her own experts is also relevant. Moreover, a district court cannot exclude a scientific technique as too confusing and overwhelming simply based on its conclusion that scientific techniques by their very nature confuse and overwhelm the jury. There must be something about the particular scientific technique such as its posture of mythic infallibility that makes it especially overwhelming. See id. at 1239 (overturning the district court’s exclusion of Dr. Nicholson’s meta-analysis because its conclusion that his testimony was overwhelming was based merely on its scientific nature and his credentials).
We explained in Downing that Rule 702 analysis partly incorporates Rule 403 analysis but leaves some room for Rule 403 to operate independently. See Downing, 753 F.2d at 1242. For example, we noted that a judge might use Rule 403 to exclude an expert’s critique of eyewitness testimony even though the critique met the requirements of Rule 702 if there was evidence of defendant’s guilt other than eyewitness testimony which would make efforts to criticize eyewitness testimony a waste of time.
In Daubert , the Supreme Court seems to have inverted our view that much of Rule 403 analysis conflates into Rule 702; rather the Court seems to have conflated the confusion/overwhelming impact prong of our Rule *747 702 analysis into its Rule 403 analysis. The Daubert Court did not mention the confusion/overwhelming prong when discussing Rule 702 but did provide support for application of essentially similar analysis under the rubric of Rule 403. The Court noted that Rule 403’s balancing test of the probative against the prejudicial value of evidence has a special role in cases involving expert witnesses. It stated that because expert evidence is often more misleading than other evidence, Rule 403 gives a judge more power over experts than over lay witnesses. See Daubert, — U.S. at-, 113 S.Ct. at 2798 . This, however, does not change our opinion that in order for a district court to exclude scientific evidence, there must be something particularly confusing about the scientific evidence at issue — something other than the general complexity of scientific evidence. 16
6. Procedural concerns regarding the Rule 403/702 balancing test
In Paoli I we held that Rule 403 is rarely appropriate as a basis of pre-trial exclusion, because a judge cannot ascertain potential relevance -until that judge has a virtual surrogate for a trial record. See Paoli I, 916 F.2d at 859-60 . There, many of the district court’s admissibility determinations were based in part on Rule 403; thus, it is important to determine whether the in limine hearing in the district court created the “virtual surrogate for a trial record” that we have required before exclusion is permissible. Plaintiffs argue that the in limine hearing did not create such a record because they did not have an opportunity to depose defendant’s experts ahead of time. As compared with a trial, plaintiffs assert, this made it much more difficult to impeach defendants’ experts and to bolster the claims of their own experts. We think that the in limine hearing conducted here was more than adequate as a basis for a Rule 403 determination — to hold otherwise given the extensive proceedings in the district court would be to require not a “virtual surrogate for a trial record” but a trial itself.
B. Rule 703
While Rule 702 focuses on an expert’s methodology, Rule 703 focuses on the data underlying the expert’s opinion. Under Rule 703:
The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence.
Under our case law on Rule 703, “the proper inquiry is not what the court deems reliable, but what experts in the relevant discipline deem it to be.” In re Japanese Elect Prod., 723 F.2d at 277 . See also DeLuca, 911 F.2d at 952; Paoli I, 916 F.2d at 853 . We have held that the district judge must make a factual finding as to what data experts find reliable, see id. at 853 , and that if an expert avers that his testimony is based on a type of data on which experts reasonably rely, that is generally enough to survive the Rule 703 inquiry. See DeLuca, 911 F.2d at 952.
This view has been rejected by other courts in favor of the view that the judge has to make an independent evaluation of the reliability of the data. See, e.g., Christophersen v. Allied-Signal Corp., 939 F.2d 1106, 1113-14 (5th Cir.1991), cert. denied, — U.S. —, 112 S.Ct. 1280 , 117 L.Ed.2d 506 (1992); Ealy v. Richardson-Merrell, Inc., 897 F.2d 1159, 1161-62 (D.C.Cir.), cert. denied, 498 U.S. 950 , 111 S.Ct. 370 , 112 L.Ed.2d 332 (1990). As Judge Weinstein wrote in In re “Agent Orange” Prod. Liab. Litig., 611 F.Supp. 1223, 1245 (E.D.N.Y.1985):
*748 Rule 703 permits experts to rely upon hearsay. The guarantee of trustworthiness is that it be of the kind normally employed by experts in the field.... Nevertheless, the court may not abdicate its independent responsibilities to decide if the bases meet minimum standards of reliability as a condition of admissibility. See Fed. R. Ev. 104(a). If the underlying data are so lacking in probative force and reliability that no reasonable expert could base an opinion on them, an opinion which rests entirely upon them must be excluded.
Judge Weinstein’s view is extremely persuasive, and we are free to express our agreement with it because we think that our former view is no longer tenable in light of Daubert. See Stana v. School Dist. of Pittsburgh, 775 F.2d 122 , 130 n. 8 (3d Cir.1985) (noting that “[o]ur authority to align this court’s jurisprudence with Supreme Court teaching is clear.”). Daubert makes clear for the first time at the Supreme Court level that courts have to play a gatekeeping role with regard to experts. In stating that Rule 702 is the primary locus of the gatekeeping role, the Court implies that there are at least some secondary loci in other Rules. See Daubert, — U.S. at-, 113 S.Ct. at 2795 . By requiring the judge to look to the views of other experts rather than allowing the judge to exercise independent judgment, current Third Circuit case law eviscerates the judge’s gatekeeping role with respect to an expert’s data and instead gives that role to other experts. The gatekeeping role is reduced even further by our DeLuca holding that the opinion of one expert that a type of data is reliable will generally be enough to render that data reliable.
We now make clear that it is the judge who makes the determination of reasonable reliance, and that for the judge to make the factual determination under Rule 104(a) that an expert is basing his or her opinion on a type of data reasonably relied upon by experts, the judge must conduct an independent evaluation into reasonableness. The judge can of course take into account the particular expert’s opinion that experts reasonably rely on that type of data, as well as the opinions of other experts as to its reliability, but the judge can also take into account other factors he or she deems relevant.
We think that the standard is equivalent to Rule 702’s reliability requirement — there must be good grounds on which to find the data reliable. It makes sense that the standards are the same, because there will often be times when both Rule 702 and Rule 703 apply. 17 For example, expert testimony that uses animal studies must, to be admissible for the purpose of drawing conclusions about humans, meet Rule 702’s requirement of “fit.” This requires the court to assess whether there are good grounds for concluding that the animal studies demonstrate causation in humans. But for an expert to rely on animal studies (if their admissibility in evidence is not independently established), these studies must also meet the requirements of Rule 703 — the conclusions of these studies must be data which must be of a type reasonably relied upon by experts to analyze causation in humans. If the expert’s reliance on animal studies to draw conclusions about people constitutes a methodological flaw, it is a methodological “flaw” consisting of relying on data not of a type reasonably relied upon by experts. 18 It would be *749 problematic to conclude that this testimony was rehable enough to meet Rule 702 but not Rule 703—or vice versa. Testimony that is admissible as rehable under one Rule should not be inadmissible under another when the pohcy considerations underlying the two rules are the same. 19
We therefore think that when a trial judge analyzes whether an expert’s data is of a type reasonably rehed on by experts in the field, he or she should assess whether there are good grounds to rely on this data to draw the conclusion reached by the expert. Whether experts in the field rely on this type of data will simply continue to be a part of the judge’s analysis. After all, the only purpose of Rule 703 was to supplant the (former) common law rule that experts could not (reasonably) rely on inadmissible evidence, a notion demonstrated by the Rule 703 advisory committee to be contrary to real life.
C. Standard of Review
A district court’s ruling on admissibility of evidence is reviewed for abuse of discretion, “but to the extent the district court’s ruling turns on an interpretation of. a Federal Rule of Evidence our review is plenary.” DeLuca, 911 F.2d at 944. The threshold rule is, of course, one of deference. However:
the justifications for committing decisions to the discretion of the court are not uniform, and may vary with the specific type of decisions. Although the standard of review in such instances is generally framed as “abuse of discretion,” in fact the scope of review will be directly related to the reason why that category or type of decision is committed to the trial court’s discretion in the first instance.
United States v. Criden, 648 F.2d 814, 817 (3d Cir.1981). In several other areas, we have applied a heightened abuse of discretion review. See, e.g., Brody v. Spang, 957 F.2d 1108, 1115 (3d Cir.1992) (“We review a denial of a motion to intervene as of right for abuse of discretion, although this review is ‘more stringent’ than the abuse of discretion review we apply to a denial of a motion for permissive intervention”); Harris v. Pernsley, 820 F.2d 592, 597 (3d Cir.), cert. denied, 484 U.S. 947 , 108 S.Ct. 336 , 98 L.Ed.2d 363 (1987). Under the Harris standard, a denial of intervention as of right should be reversed if the district court “applied an improper legal standard or reached a decision that we are confident is incorrect.” (internal quotation omitted); United States v. Miele, 989 F.2d 659, 667 (3d Cir.1993) (looking rigorously to see whether the district court had closely scrutinized drug quantity when the court based its decision on the statements of an addict-informant); cf. Malchman v. Davis, 706 F.2d 426, 434 (2d Cir.1983) (holding that while a court of appeals normally reviews a class action settlement for abuse of discretion “this principle [of deference] applies with less force when, as here, settlement of a class action has been negotiated before the class has been certified.”).
While evidentiary rulings are generally subject to a particularly high level of deference because the trial court has a superior vantage point to assess the evidence, see Criden, 648 F.2d at 818 , evaluating the reliability of scientific methodologies and data does not generally involve assessing the truthfulness of the expert witnesses and thus is often not significantly more difficult on a cold record. Moreover, here there are factors that counsel in favor of a hard look at *750 (more stringent review of) the district court’s exercise of discretion. For example, because the reliability standard of Rules 702 and 703 is somewhat amorphous, there is a significant risk that district judges will set the threshold too high and will in fact force plaintiffs to prove their case twice. Reducing this risk is particularly important because the Federal Rules of Evidence display a preference for admissibility. 20 See Daubert, — U.S. at -, 113 S.Ct. at 2794 .
Factors like those just discussed influence the scope of our review of a district court’s discretion. For example, where rules display a preference for a particular outcome, our review of decisions under those rules is sometimes more searching. See United States v. Pennsylvania Dept. of Env’t Resources, 923 F.2d 1071, 1074 (3d Cir.1991) (“[T]he traditional discretion of the federal courts to decide whether to hear declaratory judgment cases is not limited by Colorado River [Water Conservation District v. U.S., 424 U.S. 800 , 96 S.Ct. 1236 , 47 L.Ed.2d 483 ] [ (1976) ] and Moses H. Cone [Memorial Hospital v. Mercury Construction Corporation, 460 U.S. 1 , 103 S.Ct. 927 , 74 L.Ed.2d 765 ], [(1983)] but will be subject to the liberal interpretation to be accorded the Declaratory Judgment Act. Given our correspondingly enlarged scope of review ..., the district court’s declination to exercise jurisdiction ... is not consistent with a sound exercise of judicial discretion.” (internal quotation omitted)).- Moreover, the likelihood of finding an abuse of discretion is affected by the importance of the district court’s decision to the outcome of the ease and the effect it will have on important rights. See Marroquin-Manriquez v. Immigration and Natur. Serv., 699 F.2d 129, 134 (3d Cir.1983) (abuse of discretion will only be found in discovery if there has been interference with a substantial right or fundamental unfairness at the trial has resulted.); cf. Mathews v. Eldridge, 424 U.S. 319, 334 , 96 S.Ct. 893, 902 , 47 L.Ed.2d 18 (1976) (the procedural safeguards required by due process increase as the importance of the decision being made increases).
We acknowledge that there is arguably a tension between the substantial deference normally accorded to rulings where the trial court has a superior vantage point and the preference for admissibility of the Federal Rules of Evidence. We resolve any such tension by holding that when the district court’s exclusionary evidentiary rulings with respect to scientific opinion testimony will result in a summary or directed judgment, we will give them a “hard look” (more stringent review, cf. Brody v. Spang, 957 F.2d at 1115) to determine if a district court has abused its discretion in excluding evidence as unreliable. 21
D. The Role of Pennsylvania’s Requirement of Reasonable Medical Certainty
Pennsylvania requires experts to testify that defendant’s actions caused plaintiff’s illness with a reasonable degree of medical certainty, see Cohen v. Albert Einstein Medical Ctr., 405 Pa.Super. 392 , 592 A.2d 720, 724 (1991), appeal-denied, 529 Pa. 644 , 602 A.2d 855 (1992) (“Expert testimony is admissible when, taken in its entirety, it expresses reasonable certainty that the accident was a substantial factor in bringing about that injury. The expert need not express his opinion in precisely the same language we use to enunciate the legal standard.”) (quoting Kravinsky v. Glover, 263 Pa.Super. 8 , 396 A.2d 1349 (1979) (citation within Kravinsky omitted)). The issues presented by this appeal require us to determine whether this rule governs in federal court. Because it will facilitate our discussion of the individual evidentiary rulings if we do so in the earlier portions of the opinion, we turn our attention to the question now.
*751 If the rule conflicts with federal rules and is “rationally capable of classification” as procedural rather than substantive, then, as a federal court, the district court ignores the rule and applies federal rules instead. See Hanna v. Plumer, 380 U.S. 460, 472 , 85 S.Ct. 1136, 1144 , 14 L.Ed.2d 8 (1965). But “the determination of whether a particular evidentiary ruling involves federal procedural law or state substantive law, can be difficult. Often admissibility issues overlap with substantive concerns such as standards of proof.” Schulz v. Celotex Corp., 942 F.2d 204, 207 (3d Cir.1991).
In Schulz , we refrained from deciding whether New Jersey’s requirement that a medical expert testify with “reasonable medical certainty” was procedural or substantive, because we held that the federal rules, like the New Jersey rules, also required that, at a minimum, a physician be able to “form an opinion with sufficient certainty so as to make a medical judgment,” id. at 209 , and that this was enough to decide the particular case. Thus, while Schulz did not require the specific words “reasonable degree of medical certainty,” testimony about “possibilities” or even “strong possibilities” was not admissible (though testimony made without qualifications was admissible). Id. at 208 . The issue is framed here primarily by an instance in which Dr. Sherman stated her conclusion with more certainty than a “strong possibility” (she stated that PCBs were a “significant factor” in Amber Burrell’s diminished ankle reflexes) but was unwilling to testify that she had reached that conclusion with a reasonable degree of medical certainty. (The issue also arises with respect to other pieces of evidence.) Thus, we must decide whether experts were required to testify with a reasonable degree of medical certainty.
We conclude that they were. Although if it were purely a rule of admissibility Pennsylvania’s standard would not apply in federal court, Pennsylvania’s rule also constitutes part of the plaintiffs burden of proof. As a rule of admissibility, Pennsylvania’s standard is in conflict with Rules 702 and 703 which require a reliable methodology and reliable data but nowhere require a reasonable degree of medical certainty. See United States v. Cyphers, 553 F.2d 1064, 1072 (7th Cir.), cert. denied, 434 U.S. 843 , 98 S.Ct. 142 , 54 L.Ed.2d 107 (1977) (upholding a district court’s admission of expert testimony that was not based on reasonable scientific certainty); cf. Daubert, — U.S. at—, 113 S.Ct. at 2794 (holding that the Frye rule requiring a methodology to be generally accepted was not part of the Federal Rules of Evidence); but see Grant v. Farnsworth, 869 F.2d 1149 (8th Cir.1989), cert. denied, 493 U.S. 898 , 110 S.Ct. 252 , 107 L.Ed.2d 202 (1989) (upholding the district court’s exclusion of expert testimony where a chiropractor could not state the causes of plaintiffs disability with a reasonable degree of medical certainty).
If Pennsylvania’s rule were only a standard of admissibility in conflict with Federal Rules of Evidence Pennsylvania’s rule would be “rationally capable of classification as procedural” and the Federal Rules of Evidence would govern. See, e.g., Salas v. Wang, 846 F.2d 897 , 904-06 (3d Cir.1988) (holding that under the Federal Rules of Evidence, an expert could testify to aggregate damages and that this rule was procedural, thus trumping New Jersey’s opposite rule). However, we think that Pennsylvania’s rule is more than a requirement of admissibility, but rather it is an element of plaintiffs burden of proof. In Cohen, the Pennsylvania Superior Court stated that:
When a party must prove causation through expert testimony the expert must testify with reasonable certainty that in his professional opinion, the result in question did come from the cause alleged ... [I]f the plaintiffs ... expert cannot form an opinion with sufficient certainty so as to make a [professional] judgment, there is nothing on the record with which a [fact-finder] can make a decision with sufficient certainty so as to make a legal judgment.
Cohen, 592 A.2d at 723 (quoting Kravinsky v. Glover, 263 Pa.Super. 8 , 396 A.2d 1349 (1979)) (internal quotations omitted). Thus, the Cohen court strongly implied that, even if admissible, testimony with less than a reasonable degree of medical certainty was insufficient to prove causation. See also id. *752 (stating that this was “[t]he degree of medical certainty necessary to prove causation.”).
As part of the burden of proof, then, Pennsylvania’s rule is a substantive one, not in conflict with Federal Rules of Evidence, and thus governs in federal court. See, e.g., Herber v. Johns-Manville Corp., 785 F.2d 79, 81-82 (3d Cir.1986) (holding that New Jersey’s requirement that a plaintiff must show a “reasonable medical probability” of future injury in order to recover for that injury governed in federal court). As a result, in analyzing the testimony of Dr. Sherman and other witnesses with respect to particular plaintiffs, we will uphold the district court’s grant of summary judgment for the defendants in those instances when the plaintiffs’ expert states the he or she could not testify that a plaintiffs illness was caused by PCBs with a reasonable degree of medical certainty. We turn to application of the foregoing principles to the record in this case.
VI. THE RELATIONSHIP BETWEEN PCBs AND PLAINTIFFS’ ILLNESSES
In order to sustain their traditional tort claims, i.e. other than for medical monitoring, the personal injury plaintiffs must show that they were exposed to the chemicals released by the defendants, that these chemicals can cause the types of harm they suffered, and that the chemicals in fact did cause them harm. We begin our analysis at the last step, because this is the step at which it is easier to winnow the claims on which the plaintiffs have presented admissible testimony.
Eleven of the Kohn/Klehr plaintiffs offered Janette D. Sherman, M.D. as an expert in internal medicine and toxicology., They proposed to have Dr. Sherman testify: (1) that they had been exposed to PCBs and other toxic chemicals; (2) that the toxic chemicals had caused adverse health effects in the various plaintiffs including high blood pressure, lung disease, asthma, elevated triglycerides, aortic sclerosis and eventual death, arthritis, respiratory infections and a history of miscarriages; and (3) that exposure to these chemicals poses an increased risk for various plaintiffs of developing cancer or other illnesses in the future. These plaintiffs also proffered Dr. Sherman to show that they require regular medical monitoring. The district court excluded Dr. Sherman’s testimony holding that she lacked the requisite expertise, that her methodology was unreliable, and that the probative value of her testimony was substantially outweighed by unfair prejudice and undue delay.
Nine plaintiffs, those represented by Bruce Hanes, called Dr. G. John DiGregorio to testify that the plaintiffs’ present injuries were caused by PCB exposure, that they have an increased and reasonable fear of future injuries, and that a medical monitoring program, which Dr. DiGregorio would design, should be implemented for plaintiffs. (A2634-35). Dr. DiGregorio received both a Ph.D. in pharmacology and an M.D. from Hahnemann Medical College and is a professor in pharmacology and medicine at Hahne-mann. The district court accepted Dr. Di-Gregorio as a qualified expert but excluded his testimony regarding personal injuries as unreliable.
We will begin our analysis by evaluating Dr. Sherman’s qualifications and will reverse the district court’s exclusion of Dr. Sherman as an unqualified expert. We will next briefly examine the district court’s exclusion of the immunological tests relied' on by Dr. Sherman for some of her diagnoses and will uphold that exclusion. We will then discuss the defendants’ claims that Drs. Sherman and DiGregorio engaged in too few of the standard diagnostic techniques of “differential diagnosis,” and failed seriously to consider the possibility of alternative causes of plaintiffs’ illnesses other than exposure to PCBs. After describing the critique, we will apply Daubert to analyze generally what makes a doctor’s method of differential diagnosis so unreliable as to be within the district court’s power to exclude it. We will then apply this framework to the assessment of individual plaintiffs that Dr. DiGregorio and Dr. Sherman made. We will uphold the district court’s exclusion of the testimony of Dr. Sherman and Dr. DiGregorio for the assessment of present injuries with the exception of Dr. Sherman’s testimony regarding Bessie Cunningham and Amber Burrell.
*753 A. Dr. Sherman’s Qualifications
Dr. Sherman is a 1964 graduate of the Wayne State University School of Medicine. Following a one year internship at Wayne State University Hospital and a two year fellowship funded by the National Institutes of Health to study diabetes, she practiced internal medicine full time from 1970-76. Since that time she has apparently been engaged in the full time practice of forensic medicine. Dr. Sherman is the author of 22 articles and of the book Chemical Exposure and Disease—Diagnostic and Investigative Techniques. She served for six years on a 16 person advisory committee to the EPA that was convened in connection with the adoption of the Toxic Substance Control Act—an Act largely concerned with PCBs. She was on the consulting staff of Harper Hospital in Detroit until 1988 and is still on its correspondence staff. A. 5086-87.
The district court held that Dr. Sherman was not qualified to testify as an expert. It explained that Dr. Sherman no longer practices general internal medicine—she does not have admitting privileges at any hospital and lacks an office where she can examine patients. Since 1976 most of her work has consisted of consulting for litigation; she has been involved in 800 cases and has been on the plaintiffs side in all but two. According to the district court, out of fifty patients she has examined who have been exposed to PCBs, she cannot remember one instance in which she found that PCBs had not caused adverse health effects. A. 5124.
Moreover, the district court noted, Dr. Sherman lacks board certification in any medical specialty including internal medicine and toxicology, and admits that she is not an expert in immunology, oncology, neurology, psychiatry, dermatology, epidemiology or exposure assessment. Although Dr. Sherman represents that she became an expert in toxicology through experience, reading, writing, teaching, evaluating patients and course work, she took no courses on toxicology in medical school, and the district court concluded that it did not observe anything in her testimony to indicate that her reading was of a serious nature.
Finally, the district court credited defendants’ witnesses who testified that, during the course of her testimony, Dr. Sherman made numerous basic medical errors in the fields of immunology, internal medicine and dermatology. The court referred to several examples cited by defendants’ experts. Dr. Jordan Fink, Professor of Medicine and Chief of the Allergy Immunology Division at the Medical College of Wisconsin, testified that Dr. Sherman incorrectly excluded essential hypertension, the name for the 80% of hypertension with an unknown cause, from her list of the causes of hypertension, incorrectly stated that cells in the immune system transport skin pigment cells, and referred to “myelin cells” which do not exist. Dr. James Leyden, a professor at the University of Pennsylvania Medical School, who has twice won the gold medal from the American Academy of Dermatology, stated that Dr. Sherman incorrectly asserted that one could not distinguish between ehloracne and acne based on their appearance—a distinction which, in his view, any qualified dermatologist could easily make. Drs. Michael Wei-tekamp, Robert Morgan and Paul Roediger pointed out similar errors in Dr. Sherman’s testimony. And Dr. Fink concluded that these gaps in knowledge were “major deficits. They—these items should be right in the forefront of knowledge of any internist, any resident, any medical student.” The district court concluded that these “major deficits” removed Dr. Sherman from the category of expert.
We disagree. We hold that Dr. Sherman, while arguably a relatively poor clinician and less than fully credible witness, qualifies as an expert. She was on the consulting staff of a hospital until she moved from Michigan in 1988. Since then she has had extremely broad experience in the field of toxic substances, has written extensively in the field and has consulted for the American Legion Science Panel—a group of experts analyzing scientific issues, such as dioxin poisoning, that affect veterans. The fact that most of her work since 1976 has been for plaintiffs in litigation may undermine her credibility but does not eradicate her expertise. For litigants to have access to experts, it may be *754 necessary for some experts to concentrate on litigation.
Dr. Sherman apparently made substantive mistakes in her testimony, but that testimony also demonstrated significant familiarity with the literature on PCBs; indeed her testimony in that area, which is central to this case, was not substantially challenged. Of course, Dr. Sherman was not merely testifying about PCBs but about the relation of PCBs to diseases in particular plaintiffs. In order to testify about this relationship, she needed to know about the various diseases, and it is mostly in this area that she made mistakes. But in a deposition lasting four days, it is hardly surprising that there were a few technical errors. Indeed, many of Dr. Sherman’s mistakes seem minor and unlikely to affect her ability to express an opinion (e.g. use of the word “thymus” instead of “thyroid,” use of the terms “myelin cells” instead of “myelin containing cells”).
Others of Dr. Sherman’s “mistakes” are hard to judge. For example, her reference to the movement of skin pigment referred to pigment that developed in nails and skin as a result of exposure to PCBs rather than to general. skin pigment (Á791, A5114) — and whether this type of pigment moves is unclear. However, some of the mistakes Dr. Sherman made, such as her contention that acne and ehloracne look the same, seem significant for diagnostic purposes. 22 Moreover, the number of mistakes she made was quite significant.
Nonetheless, the mistakes Dr. Sherman made do not render her unqualified as an expert for purposes of Rule 702. If the liberal standard of Rule 702 allows an engineer who teaches auto mechanics to testify in a products liability action about tractors, see Hammond v. International Harvester Co., 691 F.2d 646 (3d Cir.1982), supra at pp. 741-42, it surely allows a trained internist-who has spent significant time reviewing the literature on PCBs to testify as to whether PCBs caused illness in plaintiffs. In light of Hammond , Rule 702 does not require plaintiffs to hire separate experts on the relationship between PCBs and diabetes, PCBs and chlorac-ne, PCBs and cancer etc.; it is likely that none such exist, and the cost would be prohibitive. Under our liberal standards for qualifications of experts, the district court’s decision that Dr. Sherman was not a qualified expert was an abuse of discretion.
B. Rule 70S and Dr. Sherman’s Immunological Tests
The district court also held that some of the data on which Dr. Sherman relied, immunological test results from a chemical injury kit, were not the type of data reasonably relied-upon by experts in the field under Rule 703. The district court based its holding on the fact that the lab work was performed by an unaccredited laboratory and the fact that two of defendants’ experts testified that the tests were not important, would probably be more misleading than helpful, and had been misinterpreted. Plaintiffs cite Dr. Sherman’s testimony for the proposition that she did not rely on these tests. Defendants respond that these tests were often the only basis she cited for her conclusion that PCBs had caused plaintiffs’ injuries.
We will analyze the importance of the tests to Dr. Sherman’s testimony when we analyze her testimony with respect to the particular plaintiffs. See supra p. 749 n. 19 (explaining what a court should do when an expert testifies based in part on data deemed unreliable under Rule 703). However, since the plaintiffs do not defend the tests themselves, there is no real dispute that they may not be the basis for any of Dr. Sherman’s conclusions. If Dr. Sherman is allowed to testify, she will not be allowed to rely on these tests.
*755 C. Adequacy of Dr. Sherman’s and Dr. DiGregorio’s Use of Differential Diagnosis in Evaluating Causation of Present Illnesses
1. The district court’s critique
The district court also excluded Dr. Sherman’s testimony under the helpfulness prong of Rule 702. It believed defendants’ seven witnesses who opined that Dr. Sherman did not employ differential diagnosis, the basic method of internal medicine. Dr. Fink testified that an internist should inquire of the patient about his or her complaint(s), the history of the complaint(s), any other illnesses the patient might have, the patient’s occupation, and habits such as smoking. In his view, the internist should then use differential diagnosis to rule out possible alternative diseases and causes of these diseases by outlining the possibilities and then narrowing them down based on the doctor’s experience. For example, even if plaintiffs were exposed to PCBs and PCBs are dangerous, a doctor employing proper methodology could not conclude that PCBs were a substantial factor in the cause of an illness without thinking about other potential causes.
More particularly, according to Dr. Fink, before a doctor could reasonably conclude that PCBs contributed to an individual’s lung cancer, she would have to ask whether the individual was a smoker, and if that individual was a smoker, she would have to take steps to determine whether the individual’s lung cancer was more consistent with exposure to smoking or PCB exposure or both. She would also have to consider the history of the person’s exposure to both PCBs and cigarettes, and would have to perform laboratory tests and physical examinations to determine the likely cause(s).
According to the district court, Dr. Sherman failed to employ any of the ordinary methods doctors use to narrow down the cause of disease. She did not order standard laboratory tests to be performed on plaintiffs (ordering only unhelpful immunological tests from an unaccredited laboratory), physically examined only two of the thirteen plaintiffs about whom she offered opinions, 23 and took medical histories only from the two plaintiffs she examined physically. 24 The district court noted that Dr. Fink testified that it is standard medical practice to take medical histories and conduct physical examinations, and that it would not be appropriate to base a patient’s care solely on medical records from the patient’s prior physicians. Similarly, Dr. Weitekamp testified that without going through the entire process outlined above, it is difficult to make a complete diagnosis, and that Dr. Sherman did not perform a differential diagnosis.
The district court also relied on the fact that Dr. Sherman did not even consider alternative causes for various diseases. The court pointed to the testimony of Dr. Fink and Dr. Roediger, both of whom implied that *756 the real problem with Dr. Sherman’s methodology was not that she was unaware of possible alternative causes but that she did not really consider them. 25 Defendants note that, despite her testimony that she had employed differential diagnosis and had considered alternative causes, when asked about specific findings of causation, Dr. Sherman acknowledged that she had not looked at nor ruled out alternative causes for plaintiffs’ illnesses including high blood pressure, stomach cancer, asthma, episcleritis or anemia. Moreover, Dr. Roediger testified that, “I do not feel that the concept of ruling out the commonest causes of diseases was ever displayed in her testimony in a fashion that would portray her as a competent internist.” This was because she did not give evidence of having considered the common causes of many of the diseases she diagnosed. On this basis, the district court held that Dr. Sherman’s methodology was unreliable.
For similar reasons, the district court excluded the testimony of Dr. DiGregorio. In some ways Dr. DiGregorio’s methodology was more complete than was Dr. Sherman’s. He took a limited medical history, of each plaintiff — asking about their family medical history. He also had a blood test and a fat biopsy performed on each plaintiff to test for PCBs. Nonetheless, in other ways Dr. Di-Gregorio’s analysis was more limited than that of Dr. Sherman. Dr. DiGregorio did not even look at medical records of the plaintiffs much less examine them. With selective exceptions, Dr. DiGregorio’s methodology was to have each plaintiff mark their symptoms on a checklist, answer questions about their family medical history, discuss the answers they had given on the checklist, and obtain blood and fat samples to be tested for PCBs.
The district court applied the testimony offered by defendants about Dr. Sherman to the methodology of Dr. DiGregorio. 26 It found that differential diagnosis which is used to rule in or out alternative causes is the “hallmark” of internal medicine and that Dr. DiGregorio had not employed it. The court found that Dr. DiGregorio had intended to use differential diagnosis but had not performed the examinations and tests that he as well as defendants’ experts thought were necessary to render “a correct, final diagnosis.” The court found that Dr. DiGregorio admitted that, until he physically examined plaintiffs, he would not even be able to know if they suffered any injuries caused by PCBs. Yet Dr. DiGregorio did not perform these tests or examinations in the four years between his 1988 deposition and his 1992 deposition, although he had testified in 1988 that he planned to perform these tests and exams and that they' could be completed in six months.
According to the district court, Dr. DiGre-gorio’s failure to perform these tests not only left him with little information about plaintiffs’ actual injuries, it left him with little information about causation. The court cited examples of Dr. DiGregorio being unaware of activities engaged in by plaintiffs that were *757 possible alternative causes of their injuries, such as excessive alcoholic drinking as an alternate cause of Craig Brown’s elevated LDH, and taking the birth control pill Or-tho-Novum as an alternate cause of Cathlene Brown’s bruising. The court observed that Dr. DiGregorio had simply concluded that all of the plaintiffs’ symptoms were caused by exposure to PCBs “until proven otherwise.”
As a result of the foregoing, the court found that Dr. DiGregorio’s methodology was incomplete and therefore would not be helpful to the jury under Rule 702. In the court’s view, by failing to consider alternative causes, Dr. DiGregorio’s testimony would force the jury arbitrarily to choose one cause from many possible causes and, because Dr. DiGregorio did not examine plaintiffs’ to determine that they actually had injuries, his methodology was rendered incomplete and unreliable. Finally, because Dr. DiGregorio failed to perform physical exams or many tests, the court determined that the type of data he relied on was not a type experts reasonably rely on under Rule 703. It noted in conclusion that “[t]he only opinion that Dr. DiGregorio is consistent in holding throughout his deposition is that the Plaintiffs’ anxiety, fear of cancer, depression and abnormal lab values were caused by exposure to PCBs.”
2. The plaintiffs’ response
The plaintiffs attempt to revive the methodologies of Drs. Sherman and DiGregorio by arguing that the requirements of differential diagnosis are not nearly as rigorous as the district court found them to be. Plaintiffs point out that Dr. Sherman testified at the in limine hearing that she had used differential diagnosis since she was in medical school, and that she had done so here even with regard to those plaintiffs whom she did not examine. More particularly, she looked at medical records, which contained the results of physical exams performed by other physicians (and medical histories taken by other physicians), and at illnesses across the population, and then applied the medical literature to narrow down the cause. Dr. Sherman also testified that she did consider possible alternative causes before reaching her conclusions.
Similarly, while it is true that Dr. DiGrego-rio testified that in order to render a final opinion he would have to perform more tests, he also testified that, absent further evidence, he had enough information to conclude with a reasonable degree of medical certainty that PCBs were the cause of plaintiffs illnesses. He explained:
When a doctor does a history and there’s a cause of a problem, whether it’s pneumonia, pulmonary tract infection, the differential diagnosis — the leading differential diagnosis at the time is what the doctor suspects is the immediate cause of the problem. And it’s up to the doctor to proceed after that point and to prove either by physical examination or more tests whether his diagnosis is correct. So, as far as I’m concerned the symptoms that these patients gave me in the history are due to PCB exposure. Until I have the opportunity to continue on with my investigation they all will have, within a reasonable degree of medical certainty, that type of symptom.
Thus, he concluded, at least as a preliminary manner, that until performance of physical exams or more tests, PCBs were the cause of plaintiffs’ illnesses. And in reaching this conclusion, he stated that he followed normal methodology — “[t]he differential diagnosis you make with whatever information you have at the time. What you do now is to prove your diagnosis.”
In essence, plaintiffs’ argument is that what Dr. DiGregorio was saying was that differential diagnosis is an ongoing process of making judgments about causation and then adapting those judgments as new information is acquired. Dr. DiGregorio then, was asserting that, when he made his “preliminary” differential diagnosis, he had sufficient information (his knowledge of plaintiffs’ exposure to PCBs, his knowledge of the effects of PCBs, and the medical histories he took) reliably to conclude that PCBs were more likely than not a cause of plaintiffs’ illnesses even after considering other possibilities. Thus, the question becomes whether the district court abused its discretion in concluding *758 that Dr. Sherman and Dr. DiGregorio’s methodology of assessing causation was unreliable because of the limited information they had gathered.
3. Rule 702 Analysis
The defense experts’ criticisms of the methodologies of Drs. Sherman and DiGre-gorio were specific criticisms of the manner in which they each engaged in differential diagnosis (to the point where they asserted that Drs. Sherman and DiGregorio had not even employed differential diagnosis). Unfortunately, most of the Daubert factors— testability, general acceptance, peer review, and degree of production of errors, see supra at 742 n. 8, are of only limited help in assessing whether the methodologies of Drs. Sherman and DiGregorio are reliable (i.e., scientifically valid). We find that the final Daubert factor, the existence of standards controlling the technique’s operation, is slightly more helpful in deciding whether the methodologies of the plaintiffs’ doctors are reliable.
While applying the first Daubert factor, differential diagnosis can be considered to involve the testing of a falsifiable hypothesis (e.g. that PCBs caused a plaintiffs cancer) through an attempt to rule out alternative causes, that methodology involves far more elements of judgment than does a scientific study attempting to test a more general scientific proposition. While an important aspect of assessing scientific validity (and therefore evidentiary reliability) is the ability of other scientists to test or retest a proponent’s theory, differential diagnosis involves assessing causation with respect to a particular individual. This merely makes it a different type of science than science designed to produce general theories; it does not make it unreliable science. Although in terms of the remaining Daubert factors, differential diagnosis generally is a technique that has widespread acceptance in the medical community, has been subject to peer review, and does not frequently lead to incorrect results, it is a method that involves assessing causation with respect to a particular individual. As a result, the steps a doctor has to take to make that (differential) diagnosis reliable are likely to vary from case to case; the information a doctor needs in order to reliably assess the cause of a patient’s lung cancer is often very difficult from the information needed to assess the cause of a patient’s back or heart trouble.
Thus, although differential diagnosis is a generally accepted technique, no particular combination of techniques chosen by a doctor to assess an individual patient is likely to have been generally accepted. But unlike a methodology used in conducting a scientific study, lack of general acceptance is not a sign of unreliability, it is merely a result of the fact that the medical community will rarely have considered the reliability of a particular process of differential diagnosis used in an individual case. Nor is it likely that the particular combination will have been published and subject to peer review, because a particular version of differential diagnosis will rarely be of general interest to the medical community. However, to the extent that a doctor utilizes standard diagnostic techniques in gathering this information, the more likely we are to find that the doctor’s methodology is reliable. For these reasons, we must be flexible in conducting our Daubert inquiry.
We first note that defendants presented evidence that the methodologies of Drs. Sherman and DiGregorio were not generally accepted. Defendants’ experts explained that a reliable differential diagnosis generally requires a physical examination of the patient, a review of medical records, taking a medical history and conducting of laboratory tests, and always requires careful consideration of alternative causes. We agree with the defendants that performance of physical examinations, taking of medical histories, and employment of reliable laboratory tests all provide significant evidence of a reliable differential diagnosis, and that their absence makes it much less likely that a differential diagnosis is reliable. 27 We reach *759 this conclusion in part based on the persuasive testimony of defendants’ experts that these techniques are generally accepted as a standard part of differential diagnosis, and that such techniques significantly reduce the likelihood of erroneous results. We also base our conclusion on the fact that all of the experts agree that at the core of differential diagnosis is a requirement that experts at least consider alternative causes — this almost has to be true of any technique that tries to find a cause of something. Moreover, performance of standard diagnostic techniques provides prima facie evidence that a doctor has considered such causes and has attempted to test his or her initial hypothesis as to cause.
But, we agree with the plaintiffs that a doctor does not always have to employ all of these techniques in order for the doctor’s differential diagnosis to be reliable. Dr. Di-Gregorio testified that in reaching his conclusions, he followed normal methodology— “[t]he differential diagnosis you make with whatever information you have at the time.” What Dr. DiGregorio was saying was that differential diagnosis is an ongoing process of making judgments about causation and then adapting those judgments as new information is acquired. He was asserting that even without examining the plaintiffs or conducting laboratory tests, he had sufficient information (his knowledge of plaintiffs’ exposure to PCBs, his knowledge of the effects of PCBs, and the medical histories he took) to reliably conclude that PCBs were more likely than not a cause of plaintiffs’ illnesses even after considering other possibilities. 28
Put differently, we agree with the view of Drs. Sherman and DiGregorio that sometimes differential diagnosis can be reliable with less than full information, and, to the extent that the district court concluded otherwise, we hold that it abused its discretion. To provide a simplistic example, imagine a patient who comes in with medical records that include x-rays showing a fractured arm and who tells the doctor that he hurt the arm in a biking accident; the doctor could reliably conclude that the patient had a fractured arm caused by a biking accident even without physically examining the patient or taking a *760 medical history. 29 The biking accident is so much more likely to have been the cause of the fracture than anything else that there is no need to examine alternatives. See Peteet v. Dow Chem. Co., 868 F.2d 1428, 1431-32 (5th Cir.), cert. denied, 493 U.S. 935 , 110 S.Ct. 328 , 107 L.Ed.2d 318 (1989) (allowing a doctor to testify as to causation based on an examination of medical records, of the plaintiffs deposition, and of the medical literature). 30
In light of the foregoing discussion, we conclude that where Dr. Sherman or Di-Gregorio offered an opinion as to the source of a party’s illness, the district court abused its discretion in excluding that opinion under Rule 702 unless either (1) Dr. Sherman or DiGregorio engaged in very few standard diagnostic techniques by which doctors normally rule out alternative causes and the doctor offered no good explanation as to why his or her conclusion remained reliable, or (2) the defendants pointed to some likely cause of the plaintiffs illness other than the defendants’ actions and Dr. Sherman or DiGrego-rio offered no reasonable explanation as to why he or she still believed that the defendants’ actions were a substantial factor in bringing about that illness.
In response, of course, the doctor could offer an explanation based on the plaintiffs’ medical history, laboratory tests or any other reliable source. 31 Cf. Cantrell v. GAF *761 Corp., 999 F.2d 1007 , 1012-13 (6th Cir.1993) (applying Daubert to uphold the admissibility of the testimony of a doctor that asbestos had caused the plaintiffs’ laryngeal cancer based on the unusually high incidence of this cancer among the plant workers despite the fact that the defendants had pointed to smoking as a possible alternative cause of the cancer). Thus, to avoid exclusion of his or her opinion, Dr. Sherman or DiGregorio did not have to conduct every possible test to assess whether his or her view was correct so long as he or she employed sufficient diagnostic techniques to have good grounds for his or her conclusion that the plaintiffs illness was caused by PCBs.
The conclusion that the opinion of a doctor who has engaged in few standard diagnostic techniques should be excluded unless the doctor offers a good justification for his or her conclusion is supported by the case law. Cf. In re Agent Orange Prod. Liab. Litig., 611 F.Supp. 1223, 1234-38, 1246-47 (E.D.N.Y.1985) (excluding testimony by a doctor, based on his evaluation of form statements signed by the plaintiffs in which they indicated that they had been exposed to Agent Orange and then checked off their symptoms, that Agent Orange had caused plaintiffs’ injuries at least absent evidence of an alternative cause); Arnold v. Loose, 352 F.2d 959 (3d Cir.1965) (holding that Pennsylvania law precludes an expert from testifying based on guess or conjecture, and thus that the district court had properly excluded a doctor’s opinion that an individual had lapsed into a diabetic coma before an accident when the doctor had failed to rule out the alternative possibility that the brain injury suffered during the accident had caused the coma); Viterbo v. Dow Chem. Co., 826 F.2d 420 (5th Cir.1987) (excluding doctor’s testimony that the defendant had caused plaintiffs illness, because the doctor had relied on a medical history that omitted important information, the doctor had admitted the ailments could have a number of causes, and there was no good evidence that the" defendant had caused these types of illnesses). 32
We acknowledge that some of these cases can be read to indicate that an expert’s opinion on causation should always be excluded absent employment of standard diagnostic techniques to rule out alternative causes. See also Carroll v. Litton Sys., Inc., 1990 WL 312969 , *18, 19-20, 20,1990 U.S.Dist. LEXIS 16833, *49, 53, 55 (W.D.N.C.1990) (excluding testimony of medical doctors where they failed to employ differential diagnosis and *762 rule out alternative causes). However, for the reasons explained above, we think that there will be some cases in which a physician can offer a reliable differential diagnosis without examining the patient, looking at medical records, taking a medical history, and performing laboratory tests.
We also think that the view of defendants’ experts that a physical examination and review of medical records are integral parts of differential diagnosis is not based solely on their claim that these diagnostic techniques are necessary to assess causation but also on a view that these techniques are necessary to determine what illness a patient has contracted in the first place. In In re “Agent Orange”, 611 F.Supp. at 1246 (E.D.N.Y. 1985), Judge Weinstein faced a similar situation. In the end, he excluded a doctor’s testimony as to causation, where the evidence of the patients’ illnesses stemmed solely from a checklist they had filled out. He explained, “here we have statements of problems ranging from baldness to the most serious cancers.... While courts allow reliance on patient statements, they are based upon a personal history corroborated by medical records, a physical examination and medical tests.” Id. He added, “[h]ere, the usual inducement to candor with a physician — the hope of successful treatment or diagnosis— was wholly lacking. Cf. Fed.R.Evid. 803(4). Instead, plaintiffs had every incentive to be overinclusive in describing their symptoms....” Id.
Judge Weinstein was correct. We do not doubt the propriety of a medical report prepared just for litigation purposes, but a physician who evaluates a patient in preparation for litigation should seek more than a patient’s self-report of symptoms or illness and hence should either examine the patient or review the patient’s medical records simply in order to determine that a patient is ill and what illness the patient has contracted. Thus, we are satisfied "that where Drs. DiGregorio or Sherman based their conclusion as to a plaintiffs symptoms solely on the plaintiffs self-report of illness in preparation for litigation, the district court acted within its discretion in excluding the testimony as based on an unreliable source of information.
On the other hand, we think that evaluation of the patient’s medical records, like performance of a physical examination, is a reliable method of concluding that a patient is ill even in the absence of a physical examination. It is hornbook law that medical records are admissible for such a purpose — “[a]s to hearsay symptoms told by third persons, ..., where the information is that of an attending nurse or physician having personal observation and an interest in learning and describing accurately, there seems to be every reason for admitting testimony based in part on this.” 3 John Henry Wigmore, Wig-more on Evidence § 688(4) (Chadbourn ed. 1970). See also Cohen v. Albert Einstein Medical Ctr., 405 Pa.Super. 392 , 592 A.2d 720 (1991) (holding that the testimony of a doctor who relied on medical records without conducting a physical exam was admissible proof that the plaintiff suffered from a psychiatric disorder); Peteet v. Dow Chem. Co., 868 F.2d 1428, 1432 (5th Cir.1989) (allowing testimony of a doctor who had not physically examined the plaintiff and noting that, “[a] personal examination of the person or object of the expert’s testimony is not required under Fed. R.Evid. 703”). Hence, we think that generally, a doctor only needs one reliable source of information showing that the plaintiff is ill and either a physical examination or medical records will suffice — but the doctor does need at least one of these sources. .
Moreover, as we explained above, in order to establish causation sufficiently reliable for us to overturn the district court’s decision as an abuse of discretion, where Drs. Sherman or DiGregorio engaged in few of the standard procedures of differential diagnosis, they had to offer a good explanation as to why their conclusion remained reliable. Where they did employ such standard techniques, they still had to offer such an explanation if the defendants pointed to some likely cause of the plaintiffs illness other than the defendants’ actions.
We now apply these principles to the opinions of Drs. DiGregorio and Sherman regarding particular plaintiffs.
*763 a. The opinion of Dr. DiGregorio
As we have noted, Dr. DiGregorio did not examine the plaintiffs or review their medical records; he simply relied on their answers to a questionnaire he gave them. Under the rule we have articulated above, see supra pp. 763-64, Dr. DiGregorio could not even reliably conclude that the plaintiffs had any illness. In fact, Dr. DiGregorio admitted that without examining the plaintiffs he could not state to a reasonable degree of medical certainty that they had been injured as a result of exposure to PCBs. We now uphold the district court’s decision to exclude his testimony as within that court’s discretion. Dr. DiGregorio simply lacked the foundation to make the judgments he did. For that reason alone, the district court was correct to grant summary judgment against the Hanes plaintiffs on their personal injury claims.
Moreover, inasmuch as he did not engage in standard diagnostic techniques, we presume that Dr. DiGregorio’s conclusions as to causation were unreliable unless he offered good grounds for concluding that they were correct. See supra pp. 761-62. But Dr. DiGregorio stated that his preliminary assumption was that all of the plaintiffs’ injuries that they checked off in the questionnaire were caused by PCBs, and he did not explain what justified this assumption. For example, Dr. DiGregorio testified that PCBs caused Cathlene Brown’s bruising but admitted that the birth control pills she took could also have caused the bruising. He explained that in order to confirm his diagnosis he would have to “investigate with her when did she start the bruising, how long has she been bruising, does she have any bruising now, and how long has she been on the contraceptives,” but noted that his diagnosis would remain unchanged until he could conduct this investigation. Dr. DiGregorio nonetheless provided no reason to explain why he thought that PCBs were more likely to have caused Brown’s bruising than contraceptives. 33 His analysis of other Hanes plaintiffs was similarly flawed, as explained in the margin.
Presumably, Dr. DiGregorio’s general reasoning was that- all of the plaintiffs had significant exposure to PCBs, that PCBs are a known cause of the illnesses of the various plaintiffs, and that, in the absence of any indication that any plaintiff had been exposed to something more likely to cause his or her illness than PCBs, it was reasonable to conclude that PCBs were a likely cause. But Dr. DiGregorio did not point to any evidence showing that PCB exposure was so likely to produce the type of illnesses the plaintiffs *764 had in comparison to other possible causes to which plaintiffs had likely been exposed that it was rehable to conclude that PCBs were the cause without further analysis. We think that the district court essentially, and properly, read Dr. .DiGregorio’s testimony as showing that his opinion that PCBs caused plaintiffs’ illnesses was only a hypothesis which he had yet to attempt to verify or disprove by subjecting it to the rigors of scientific testing. Thus, the district court properly excluded Dr. DiGregorio’s testimony with respect to the Hanes plaintiffs’ personal injury claims and correctly granted summary judgment against them on these claims.
b. The opinion of Dr. Sherman
Unlike Dr. DiGregorio, Dr. Sherman reviewed the medical records of all of the plaintiffs on whose behalf she testified (the Kohn/ Klehr plaintiffs). Moreover, she physically examined two plaintiffs (and took their medical history). Thus, she had a rehable source of information that each of the plaintiffs was actually ill. Dr. Sherman also had information on the history of PCBs in the Paoli area, the level of PCBs in plaintiffs’ soil, some evidence of the level in their blood (for many plaintiffs), and the medical histories of the plaintiffs that were contained in their medical records. Nonetheless, with respect to those plaintiffs whom Dr. Sherman did not examine, she had little, if any data with which to rule out alternative causes of their illnesses. She did not take medical histories herself. There is no evidence in the record showing the extent to which medical histories were present in the plaintiffs’ medical records, and she performed no rehable laboratory tests.
The district court excluded Dr. Sherman’s testimony as a whole, on the grounds that her testimony demonstrated that she never considered alternative causes and always found PCBs to be the cause. If the district court was correct, then we would likely uphold its exclusion even if the defendants did not point to specific alternative causes that Dr. Sherman failed to consider for those plaintiffs whom she did examine. Given the number of plaintiffs in this case, we think that if there were no evidence that Dr. Sherman ever considered alternative causes, it would be reasonable for the district court to have presumed that she did not consider alternative causes even for those plaintiffs whom she examined and for whom she had significant data about alternative possibilities.
However, as we read Dr. Sherman’s testimony, when she said that she did not rule out other causes, that does not necessarily mean that she failed to consider them, but rather only that she did not rule them out as a possible cause or a partial cause in combination with PCBs, which she concluded were one significant factor in plaintiffs’ illnesses. More particularly, where defendants point to evidence that Dr. Sherman acknowledged failing to rule out alternative causes, Dr. Sherman was merely admitting that there might have been other causes while stating that PCBs were a significant cause.
Plaintiffs point to several instances in which Dr. Sherman specifically considered alternative causes. She specifically considered Charles Stanbach’s smoking as a possible cause of his esophageal cancer before ruling it out based on when he had stopped smoking and the types of changes he had in his cells. Moreover, she declined to say with a reasonable degree of medical certainty that the redness around George Burrell’s eyes had been caused by PCB exposure even though PCBs were associated with such redness in the literature — she explained that she could not rule out alternative causes. For similar reasons she refused to say that Priscilla Burrell’s headaches were caused by PCBs, and she opined that Monica Hilton’s skin reaction was caused by poison ivy rather than exposure to PCBs. Dr. Sherman further testified that she could not state with a reasonable degree of medical certainty that PCBs caused ringing in the ears even though three of the plaintiffs complained of such ringing.
In view

[Text truncated at 120,000 characters. The full text is on the page linked above.]

---

Source: Frix Law Library, https://www.frixlaw.com/law-library/cases/6932215. Public record. Not legal advice.