Weight of the Evidence in Science and in Law

woe to that man by whom the offense cometh”

         Matthew 18:7

Weight of the evidence (WOE) has cropped up again in recent trial and appellate court proceedings involving the admissibility of scientific expert witness opinion testimony. With some consistency, the WOE approach advocated is vacuous. The proponents of WOE do not specify what type of evidence is considered, whether all evidence was considered, or how competing and conflicting evidence was weighed.

Interpreted sympathetically, WOE might be taken to mean that “scientific judgment” was exercised with respect to causal inference, without describing exactly what was done. Although sympathetic, this interpretation renders the purported methodology meaningless. WOE-ful scientists might just as well say that they used scientific method. Not surprisingly, WOE is absent from virtually all major epidemiology textbooks

Despite the vacuity of WOE, or because of it, some lawyers, who constitute the lawsuit industry, are particularly fond of WOE.1 Expert witnesses who support the lawsuit industry have defended their “right” to inflict WOE on the litigation system, tooth and nail.2

Carl Cranor, a philosophy professor and a hired expert witness in litigation for plaintiffs’ counsel, has written about WOE and attempted to defend WOE as a scientific methodology. Cranor has caricaturized criticisms of WOE, including mine, by suggesting that the International Agency for Research on Cancer’s use of WOE rebuts my suggestion that WOE is no method at all.3 Cranor’s defense fails, however, because IARC’s method, for all its deficiencies, never invokes a method mired in WOE.

Perhaps the Lawsuit Industry likes WOE as much as it likes the equally vague term, “link.” WOE frees them from the requirement of any meaningful methodology, which means that any conclusion is possible. Under WOE, any conclusion can survive gatekeeping as an opinion. WOE frees the putative expert witness from the need to consider the quality of research. WOE-ful authors such as Carl Cranor invoke WOE or seek to inflict WOE without mentioning the crucial “nuts and bolts” of scientific inference, such as concepts of

  • Internal and external validity
  • Assessment of random error
  • Assessment of known and residual confounding
  • Known and potential threats to validity in
  • Appropriate methods of systematic review
  • Appropriate synthesis across studies, such as systematic review and meta-analysis

These important concepts are lost in the miasma of WOE.

In the published scientific literature, it is a commonplace that WOE is either poorly or not defined and specified. The phrase is vague and ambiguous; its use, inconsistent.4  Even authors sympathetic to the WOE mission have reluctantly concluded that the term is most often used in a way that “does not lend itself to transparency or repeatability except in simple cases.”5

Another reason that WOE resonates so strongly with the Lawsuit Industry is that having expert witnesses proclaim WOE as their methodology permits trial counsel to claim that the proffered opinions are immune to gatekeeping because, after all, weight-of-the-evidence questions are for the jury. Lawyers learn early on about WOE factual issues in appellate review of a wide variety of evidentiary and sufficiency issues in criminal and civil cases.6 Unless against the great WOE, WOE questions are for the jury.

Even venerable judges fall for this semantic confusion. In 1995, the Second Circuit, before the major revision of Rule 702, in 2000, noted that in discharging their gatekeeping role, trial judges do not assume:

“‘the role of St. Peter at the gates of heaven, performing a searching inquiry into the depth of an expert witness’s soul’ that would ‘inexorably lead to evaluating witness credibility and weight of the evidence, the ageless role of the jury’.”

McCullock v. H.B. Fuller Co., 61 F.3d 1038, 1045 (2d Cir.1995) (internal citations omitted).

Of course, the expert witness’s soul is not at issue, but his methodology is. More important, however, note how the appellate court adverted to “weight of the evidence” as something that the jury must evaluate, along with witness credibility. The expert witness WOE litigation strategy deliberately trades upon the confusion between WOE in the allocation between judge and jury, and valid scientific methodology in causal inference. McCullock is proof that judges can be, and are, bamboozled by the litigation strategy.

Twenty years after McCullock, federal appellate judges are still falling for the deliberate confusion between legal and scientific WOE. The Ninth Circuit recently held that the reliability test of Federal Rule of Evidence 702 is:

“‘is not the correctness of the expert’s conclusions but the soundness of his methodology’, and when an expert meets the threshold established by Rule 702, the expert may testify and the fact finder decides how much weight to give that testimony. Challenges that go to the weight of the evidence are within the province of a fact finder, not a trial court judge. A district court should not make credibility determinations that are reserved for the jury.”

City of Pomona v. SQM North America Corp., 750 F.3d 1036, 1044 (9th Cir. 2014) (internal citation omitted), cert. denied, 135 S. Ct. 870 (2014). Characterizing a methodological dispute as one that “merely” concerns the “weight of the evidence” is a strategy to remove the dispute from judicial gatekeeping altogether.

Recently, the Third Circuit displayed this confusion of WOE with methodological impropriety by mischaracterizing failure to correct for multiple testing as merely an improper calculation that ordinarily goes to the weight of the evidence, not its admissibility. Karlo v. Pittsburgh Glass Works, LLC, 849 F.3d 61, 83 (3d Cir. 2017).

The Third Circuit, in Karlo, cited to a Supreme Court case that predated Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), and which did involve any Rule 702 challenge to the use of a flawed statistical analysis. In Bazemore v. Friday, 478 U.S. 385, 400 (1986), plaintiffs sued as a class for employment discrimination, and sought to show the discrimination through the use of a regression analysis. The defense challenged the plaintiffs’ regression on grounds that key variables were omitted. The Court rejected a sufficiency challenge to a finding of discrimination in plaintiffs’ class action, and noted:

Normally, failure to include variables will affect the analysis’ probativeness, not its admissibility.”

The lesson of the last two decades of judicial gatekeeping is that methodological infirmity will affect both probitiveness and admissibility7. Courts cannot escape their important gatekeeping duties by shifting their responsibility to juries under the guise of WOE.

2 See Schachtman, “Desultory Thoughts on Milward v. Acuity Specialty Products,” (Oct. 2015).

3 Carl F. Cranor, Toxic Torts: Science, Law, and the Possibility of Justice 146 (2d ed. 2016) (citing and selectively quoting from Schachtman, WOE-fully Inadequate Methodology – An Ipse Dixit By Another Name” (May 1, 2012)).

4 See Charles Menzie, Miranda Hope Henning, Jerome Cura, Kenneth Finkelstein, Jack Gentile, James Maughan, David Mitchell, Stephen Petron, Bonnie Potocki, Susan Svirsky & Patti Tyler, “A weight-of-evidence approach for evaluating ecological risks; report of the Massachusetts Weight-of-Evidence Work Group,” 2 Human Ecological Risk Assessment 277, 279 (1996) (“although the term ‘weight of evidence’ is used frequently in ecological risk assessment, there is no consensus on its definition or how it should be applied”); Sheldon Krimsky, “The weight of scientific evidence in policy and law,” 95 Am. J. Pub. Health S129 (2005) (“However, the term [WOE] is applied quite liberally in the regulatory literature, the methodology behind it is rarely explicated.”); V. H. Dale, G.R. Biddinger, M.C. Newman, J.T. Oris, G.W. Suter II, T. Thompson, et al., “Enhancing the ecological risk assessment process,” 4 Integrated Envt’l Assess. Management 306 (2008) (“An approach to interpreting lines of evidence and weight of evidence is critically needed for complex assessments, and it would be useful to develop case studies and/or standards of practice for interpreting lines of evidence.”);  Douglas L. Weed, “Weight of Evidence: A Review of Concept and Methods,” 25 Risk Analysis 1545 (2005) (noting the “lack of definition of the term weight of evidence, multiple uses of the term and a lack of consensus about its meaning, and the many different kinds of weights, both qualitative and quantitative which can be used in risk assessment”); R.G. Stahl Jr., “Issues addressed and unaddressed in EPA’s ecological risk guidelines,” 17 Risk Policy Report 35 (1998) (noting that U.S. Environmental Protection Agency’s guidelines for ecological weight-of-evidence approaches to risk assessment fail to provide guidance); Glenn W. Suter, Susan M. Cormier, “Why and how to combine evidence in environmental assessments:  Weighing evidence and building cases,” 409 Sci. Total Env’t 1406, 1406 (2011) (noting arbitrariness and subjectivity of WOE “methodology”).

5 See Igor Linkov, Drew Loney, Susan Cormier, F. Kyle Satterstrom, and Todd Bridges, “Weight-of-evidence evaluation in environmental assessment: review of qualitative and quantitative approaches,” 407 Sci. Total Env’t 5199, 5203 (2009).

6 See, e.g., People v. Collier, 146 A.D.3d 1146, 1147-48, 2017 NY Slip Op 00342 (N.Y. App. Div. 3d Dep’t, Jan. 19, 2017) (rejecting appeal based upon defendant’s claim that conviction was against “weight of the evidence”); Venson v. Altamirano, 749 F.3d 641, 656 (7th Cir. 2014) (noting “new trial is appropriate if the jury’s verdict is against the manifest weight of the evidence”).

7 David L. Faigman, Christopher Slobogin & John Monahan, “Gatekeeping Science: Using the Structure of Scientific Research to Distinguish Between Admissibility and Weight in Expert Testimony,” 110 Northwestern L. Rev. 859, 865 (2016) (“An expert economist in an employment discrimination case who admittedly fails to control for a key variable such as seniority or wage structure in a regression analysis has committed a general error that should lead to exclusion by a judge… .”).

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