Judging the Reliability of Expert Causation Opinions Based on Epidemiology Data After King v. Burlington Northern Santa Fe Railway Company: Is the Judge a Gatekeeper or a Matador

• Publication year: 2022

43 Creighton L. Rev. 1049. JUDGING THE RELIABILITY OF EXPERT CAUSATION OPINIONS BASED ON EPIDEMIOLOGY DATA AFTER KING V. BURLINGTON NORTHERN SANTA FE RAILWAY COMPANY: IS THE JUDGE A GATEKEEPER OR A MATADOR

JUDGING THE RELIABILITY OF EXPERT CAUSATION OPINIONS BASED ON EPIDEMIOLOGY DATA AFTER KING V. BURLINGTON NORTHERN SANTA FE RAILWAY COMPANY: IS THE JUDGE A GATEKEEPER OR A MATADOR?

Neal C. Stout(fn*)

TABLE OF CONTENTS

I. INTRODUCTION................................... 1050

II. BACKGROUND .................................... 1054

A.The Standard for Admissibility of Expert Testimony: Daubert Supersedes Frye.......... 1054

B.The Nebraska Supreme Court Adopts Daubert........................................ 1059

C.King v. Burlington Northern Santa Fe Railroad: How Daubert Applies to Epidemiologically Based Opinions in Nebraska.......................................1060

1.The Facts....................................1061

2.The Standard for Admissibility of Epidemiology-Based Opinions................1064

D.The Bradford Hill Causation Criteria........1066

III. ANALYSIS.........................................1069

A.Strength of Association....................... 1069

1.What Does "Strength of Association" Mean?... 1069

2.What is the Legal Significance of "Strength of Association"? .............................. 1071

B.Statistical Significance ....................... 1076

1.What Does "Statistical Significance" Mean?... 1076

2.What is the Legal Significance of "Statistical Significance"? .................... 1079

C.Consistency: Can One Positive Epidemiology Study be Enough?.............................. 1085

D.Unpublished Epidemiology Studies are Inherently Unreliable ........................... 1088

E.The King Court Did Not Set Forth a Standard for the Dose-Response Criterion ... 1091

1.The Dose-Response Gradient and the Threshold Dose.............................. 1092

a.Dose-Response Gradient.................. 1093

b.Threshold Dose.......................... 1094

c.The No-Threshold Theory is Unscientific.............................. 1096

2.The Dose-Response Issue in King............. 1098

3.Courts Do Not Require Exact Dose Measures, Just Reliable Estimates............ 1102

IV. CONCLUSION ..................................... 1105

I. introduction

In many toxic tort cases, the central issue is whether the subject toxic agent is capable of causing the plaintiff's (or the decedent's) injury in exposed populations-so-called "general causation."(fn1) Proof of general causation in toxic tort cases is often an issue because several factors obscure the causal pathway between the putative exposures and the diagnosed illness. First, in contrast to injuries caused by sudden and traumatic events (for example, a broken leg caused by a fall from a ladder), injuries caused by exposures to toxic agents often develop over long periods of time.(fn2) This extended temporality obscures the causal relationship, if any, between the exposures and the injuries. In addition, the injury is seldom unique to the subject exposure. In most toxic exposure cases, the injury at issue occurs in the general population and has many possible causes, including "idiopathic" (unknown cause), which complicates proof of the putative causal link between the subject toxic agent and the disease.(fn3) Even the allegation of exposure to the agent, or at least the duration and intensity of the exposure, may be at issue.(fn4) Hence, the subject agent's ability to cause the plaintiff's injury(fn5) under the exposure circumstances is often in genuine dispute.

Given the complexity of proving the causal pathway, a toxic tort plaintiff needs expert medical and scientific testimony to establish the link between the subject exposure and the injury.(fn6) Supporting epidemiologic data are often key, sometimes crucial, to the plaintiffs case.(fn7) Epidemiology is a branch of science focusing on the question of general causation, that is, whether a substance is capable of causing a particular disease. In contrast, specific causation addresses the issue of whether an exposure to a substance caused disease in a specific indi-vidual.(fn8) Epidemiology strives to "observe the effect of exposure to a single factor upon the incidence of disease in two otherwise identical populations."(fn9)

As is the case with all expert testimony, the trial court is charged with the duty to scrutinize the proffered opinions of epidemiology experts and other general causation experts to determine whether the opinions are sufficiently grounded in the methods of science and, if not, to exclude the unscientific opinions. In the federal courts, Rule 702 of the Federal Rules of Evidence,(fn10) the seminal Supreme Court of the United States case interpreting Rule 702, Daubert v. Merrell Dow Pharmaceuticals, Inc.,(fn11) and its progeny govern the judges' gatekeeping duty as applied to expert opinions. In Nebraska, Nebraska Revised Statutes section 27-702,(fn12) Rule 702 of the Nebraska Evidence Rules, governs the admissibility of expert opinion testimony.(fn13) Following Schafersman v. Agland Co-op,(fn14) Nebraska courts applied the federal courts' reliability criteria developed post-Daubert as well as the reliability criteria of sister state courts that have adopted the Daubert approach. Hence, the rich Daubert case law that has developed since that 1993 decision informed Nebraska courts of the required level of scrutiny for proffered expert opinions.

Pursuant to the reliability rules applied to expert testimony, toxic tort defendants often move that trial courts bar the plaintiffs' proffered causation evidence on the ground that the plaintiffs' causation experts' methods are unscientific and their opinions are unsupported by the data, rendering the proffered opinions unreliable "junk science." In King v. Burlington Northern and Santa Fe Railway Co.,(fn15) the defendant railroad filed such a motion, urging the trial court to exclude the general causation opinions offered by the plaintiff's medical expert who had cited epidemiologic data in support of his opinions.(fn16)On appeal, the Supreme Court of Nebraska addressed the standards by which the lower courts are to judge the admissibility of causation opinions based on epidemiologic data. The Nebraska Supreme Court's opinion in King, in particular the guidance it provided the lower courts in judging the reliability of expert opinions based on epidemio-logic data, is the subject of this Article.

This Article first briefly discusses the seminal cases of Daubert and Schafersman and the general guidelines these cases set forth with regard to the judicial scrutiny of proffered expert testimony. This Article then summarizes the King opinion and the guidelines the Nebraska Supreme Court provided to assist the lower courts in judging the reliability of expert opinions grounded on epidemiologic data. With that background, this Article analyzes the King guidelines in terms of the underlying scientific principles (primarily epidemiologic and toxicologic principles). In so doing, this Article compares the Nebraska Supreme Court's guidelines with criteria developed in other jurisdictions applying Daubert criteria to epidemiologically grounded opinions.

This Article concludes that the Nebraska Supreme Court's guidelines for judging the reliability of epidemiological^ based opinions are wanting. The court muffed a chance to provide the lower courts with guidance based on sound science. Instead, the court provided a set of criteria that hamstrings trial judges' ability to bar causation theories grounded on the flimsiest of epidemiologic data. As such, these flimsy guidelines lead to courts admitting scientifically unfounded opinions and, thus, allow juries of laypersons to speculate as to the cause of a plaintiffs disease, contrary to the purpose of Rule 702 of the Nebraska Evidence Rules. In brief, when faced with a motion to exclude causation opinions based on epidemiologic data, judges in Nebraska are no longer gatekeepers, but matadors artfully sidestepping to allow such opinions to charge into evidence.(fn17)

II. background

A. The Standard for Admitting Expert Testimony: Daubert Supersedes Frye

Prior to the adoption of the Federal Rules of Evidence, all federal and most state courts followed the test set forth in Frye v. United States(fn18) to determine the admissibility of scientific evidence. Under the Frye test, scientific evidence was admissible only if such evidence was based on a principle that was "sufficiently established to have gained general acceptance in the particular field in which it be-longs."(fn19) In Daubert v. Merrell Dow Pharmaceuticals, Inc.,(fn20) the Supreme Court of the United States held that the Federal Rules of Evidence, and in particular Rule 702,(fn21) superseded the Frye test.(fn22) In Daubert, the Court held that the Federal Rules of Evidence did not incorporate the Frye standard.(fn23) To the contrary, the issue in a court's determination whether to admit scientific opinion testimony is whether the expert used reliable methods.(fn24)

Daubert introduced a "new era" in the scrutiny of expert testi-mony.(fn25) Prior to Daubert, the Frye "general acceptance" test stood in as a surrogate for reliability; after Daubert, the trial judge must directly assess reliability.(fn26) The Supreme Court recognized that expert testimony could be "both powerful and...