Posted on: 9/11/2012
Eric Swan, Shook Hardy & Bacon
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In one of the most controversial Daubert rulings in recent memory, the First Circuit in Milward v. Acuity Specialty Products Inc. recently held that an expert's opinion was reliable based on a "weight-of-the-evidence" approach. 639 F.3d 11 (1st Cir. 2011), cert. denied sub nom U.S. Steel Corp. v. Milward, No. 10A1263, 2012 WL 33303 (Jan. 9, 2012). Because this approach is contrary to existing precedent and scientific principles, and will likely be coming to a court near you soon, I analyze some of the problems with the decision.
The background facts of the case were recently well summarized by Julie A. Brennan in an article published in the DRI Young Lawyers Committee's online newsletter, The Whisper. http://clients.criticalimpact.com/newsletter/newslettercontentshow1.cfm?contentid=9645&id=1080. Therefore, I only briefly summarize them here.
The Plaintiffs in Milward sued multiple Defendants alleging that Mr. Milward's exposure to benzene caused him to develop a blood cancer called Acute Promyelocytic Leukemia (APL), a rare subtype of a more general category of leukemia known as acute myeloid leukemia (AML). In formulating his opinion, Plaintiffs' expert toxicologist relied on several lines of evidence:
- The scientific consensus "that benzene can cause AML as a class";
- Literature suggesting that AML and APL both arise from stem cells;
- Evidence that both APL and AML are often accompanied by chromosomal translocations—chromosomes that have broken and improperly recombined;
- The ability of metabolites of benzene to cause chromosomal translocations by a variety of mechanisms; and
- Statistically insignificant epidemiologic studies that "suggest" a link between benzene and APL.
Based on the weight of this evidence, Plaintiffs' expert concluded that benzene is capable of causing APL. Milward, 639 F.3d at 20.
The precise contours of the weight-of-the-evidence methodology that the Court accepted are not altogether clear. According to the Court, the expert's analysis began with the nine factors identified in the Bradford-Hill criteria. From there the expert added a second layer of analysis. Under this methodology the expert must:
- identify an association between an exposure and a disease,
- consider a range of plausible explanations for the association,
- rank the rival explanations according to their plausibility,
- seek additional evidence to separate the more plausible from the less plausible explanations,
- consider all relevant available evidence, and
- integrate the evidence using professional judgment to come to a conclusion about the best explanation.
Id. at 18. The Court ultimately held that this approach was "scientifically sound and methodologically reliable" per Daubert, id. at 20, and, as a result, it reversed the lower court's exclusion of the expert's testimony.
III.Analysis of the Court's Reasoning
The Court's analysis is fraught with problems. First, and most importantly, by accepting this weight-of-the-evidence methodology, the Court subtly but profoundly shifted the Daubert inquiry from whether the evidence standing alone is sufficiently reliable scientific evidence of causation, to whether on balance the evidence supports an expert's theory. Second, the Court jettisoned one of the most accepted standards for making causal judgments—the hierarchy of evidence—and replaced it with a scientific standard that has no Daubert precedent. Finally, the Court's weight-of-the-evidence approach creates an analytic conundrum for judges: How are judges to reconcile their role as gatekeepers in cases involving weight-of-the-evidence opinions, when they are admonished that the "evaluation of the weight of the evidence…is the province of jury"? Id. at 20.
A.Altering the Daubert Inquiry
Although the Court tried to cloak its opinion in established authority, most notably the Bradford-Hill criteria, it analysis diverges significantly from both Bradford-Hill and established Daubert precedent. One of the most fundamental problems with the Court's analysis is that it shifted the Daubert inquiry from the reliability of the plaintiff's evidence vis-à-vis an external standard, to the strength of the expert's evidence vis-à-vis other competing evidence. This can be seen in steps 2-6 of the methodology, which permit the expert to rank competing plausible explanations and then weigh their relative strength. Id. at 18. Whereas Hill looked at a specific theory and asked whether there was sufficient evidence to say the association was causal, the Court's weight-of-the-evidence approach asks how the theory stacks up against competing theories.
The result is that there is no threshold level of evidence before an expert's opinion can be deemed admissible. For example, what if the available evidence consists entirely of one animal study and a single paper hypothesizing a mechanism of action? And what if no scientist had bothered to publish or conduct contrary studies? Under existing Daubert case law, most courts would find this to be insufficient evidence of general causation. Under a weight-of-the-evidence approach, it might be admissible because the "weight" of the evidence supports the conclusion. There is no doubt plaintiffs' lawyers will press this angle in future litigation.
B. Rejecting the Hierarchy of Evidence
Perhaps the most disconcerting portion of the opinion is the suggestion that there is no hierarchy of evidence in causal inference. At the end of its brief discussion of Bradford-Hill, the Court wrote: "[I]t is generally agreed that this list is not exhaustive and that no one type of evidence must be presented before causality may be inferred. For example, when a group from the National Cancer Institute was asked to rank the different types of evidence, it concluded that '[t]here should be no such hierarchy.'" Id. at 17. The full quote from the NCI panel is as follows:
6. What is the hierarchy of state-of-the-art approaches needed for confirmation criteria, and which bioassays are critical for decisions: epidemiology, animal testing, cell culture, genomics,and so forth? There should be no such hierarchy. Epidemiology, animal, tissue culture and molecular pathology should be seen as integrating evidences in the determination of human carcinogenicity.
Michele Carbon et al., Modern Criteria to Establish Human Cancer Etiology, 64 Cancer Res. 5518, 5522 (2004). The implication that there is no hierarchy of evidence in causal inference, and that tissue culture studies are as relevant as epidemiology, is patently absurd. Whatever value this may have in the context of regulatory decisions for the purpose of cancer prevention, it has no value and no precedent in tort law. See, e.g., Norris v. Baxter Healthcare Corp., 397 F.3d 878, 882 (10th Cir. 2005). (holding that epidemiology is the "best evidence"—and therefore better evidence—of causation). Indeed, if there is no hierarchy one must ask why the FDA requires clinical trials for drug approval? After all, by this logic, clinical trials are no better than in vitro studies and animal studies at showing safety and efficacy.
C.Creating an Analytical Conundrum
Lastly, the Court's weight-of-the-evidence approach creates analytic problems for courts assessing the reliability of expert testimony. The lower court noted three principle inferential leaps in the Plaintiffs' expert's analysis that it believed rendered his opinion unreliable. The First Circuit responded by saying these were of the lower courts own making, id. at 22, and went on to chastise the lower court for crossing "the boundary between gatekeeper and trier of fact." Id. According to the Court, "the alleged flaws [went] to the weight of Dr. Smith's opinion, not its admissibility." Id. Of course, this begs the question: When the methodology is expressly predicated on weighing the evidence how is a lower court to evaluate the reliability of the opinion if it cannot in fact weigh the evidence?
Whether it is called "weighing the evidence" or analyzing the inferential leaps in the expert's methodology, what the lower court did is consistent with the Supreme Court's admonition to courts in Joiner not to rely simply on the expert's ipse dixit. Gen. Elec. Co. v. Joiner, 522 U.S. 136, 146 (1997). The First Circuit recognized this and even cited the relevant language from Joiner: "following Joiner, a district court properly may exclude expert testimony if the court concludes too great an analytical gap exists between the existing data and the expert's conclusion." Id. (internal citations omitted). By likening the lower court's analysis of those gaps to an improper weighing of the evidence, the First Circuit gave ammunition to plaintiffs in future cases to liken all defense criticisms of their expert's inferential leaps to an inappropriate call to "weigh the evidence." It would have been better if the First Circuit simply acknowledged what it did: replace its judgment of the evidence for that of the lower court's.
The First Circuit was correct when it said that "[n]o matter what methodology is used, 'an evaluation of data and scientific evidence to determine whether an inference of causation is appropriate requires judgment and interpretation.'" Id. at 18 (internal citations omitted). However, it was wrong to adopt a standard that shifts the focus from a judgment of the evidence vis-à-vis an external standard, to a judgment vis-à-vis whatever information is available. It compounded this mistake by jettisoning the hierarchy of evidence, which provides a framework for making those judgments, and by characterizing the lower court's analysis of the expert's inferential leaps as an invasion on the fact finding role of the jury. In the end, the First Circuit obfuscated rather than elucidated the requirements for the admissibility of expert testimony and added more confusion to an already confusing area of law.
Eric Swan is an associate at Shook, Hardy, & Bacon LLP in the firm's Kansas City office. His practice focuses on pharmaceutical and medical device product liability defense. He has worked extensively on mass torts and expert-related issues. Eric has a bachelor's degree in Chemistry and a master's degree in Cancer Biology from the MD Anderson Cancer Center. He can be reached at email@example.com.