Bayes' Law, Sequential Uncertainties, and Evidence of Causation in Toxic Tort Cases
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University of Michigan Journal of Law Reform Volume 38 2005 Bayes' Law, Sequential Uncertainties, and Evidence of Causation in Toxic Tort Cases Neal C. Stout Peter A. Valberg Gradient Corporation Follow this and additional works at: https://repository.law.umich.edu/mjlr Part of the Courts Commons, Evidence Commons, Judges Commons, and the Science and Technology Law Commons Recommended Citation Neal C. Stout & Peter A. Valberg, Bayes' Law, Sequential Uncertainties, and Evidence of Causation in Toxic Tort Cases, 38 U. MICH. J. L. REFORM 781 (2005). Available at: https://repository.law.umich.edu/mjlr/vol38/iss4/3 This Article is brought to you for free and open access by the University of Michigan Journal of Law Reform at University of Michigan Law School Scholarship Repository. It has been accepted for inclusion in University of Michigan Journal of Law Reform by an authorized editor of University of Michigan Law School Scholarship Repository. For more information, please contact [email protected]. BAYES' LAW, SEQUENTIAL UNCERTAINTIES, AND EVIDENCE OF CAUSATION IN TOXIC TORT CASES Neal C. Stout* Peter A. Valberg** Judges are the gatekeepers of evidence. Arguably, the most difficult duty for a judicialgate- keeper is to screen the reliability of expert opinions in scientfic fields such as medicine that are beyond the ken of most judges. Yet, judges have a duty to scrutinize such expert opin- ion evidence to determine its reliability and admissibility. In toxic tort cases, the issue of causation-whetherthe alleged exposures actually caused the plaintiffs injury-is nearly always the central dispute, and determiningadmissibility of expert causationopinion is a daunting challengefor most judges. We present a comprehensive review of the courts' strug&les with the screening of scientific evidence in such cases. In addition, we prpose an approach to the screening of causation opinions based on probability science and logic. Central to this approach is Bayes'Law, a statisticaltool that courts can use to analyze the extrinsic reliability ofproffered causation testimony. We explain Bayes'Law and illustrate its potential applicationfor evaluating the reliability of medical and scientific causation testimony. All evidence is probabilistic. There are uncertaintiesattending all testimony, not only be- cause the honesty or objectivity of witnesses may be doubtfl, but also because even honest and unbiased witnesses may be mistaken in their perceptions. Reliability of causationevi- dence depends on both sensitiity and specificity of the tests used to determine causation. Hhly sensitie tests of causation reflect an ability to identify a high percentage of those with the agent-induced disease, whereas highly specific tests of causation reflect an ability to reject a high percentage of those who have the disease, but not induced by the agent at is- sue. Accoding to Bayes'Law, the reiabilityof causation opinion depends not only on the sensitivity and specificity of the tests employed by the causationexpert, but also on the base- rate of the agent-induceddisease in the population. Bayes' Law dictates that the lower the rate of the agent-induced disease in the population, the less reliable the opinion that the agent at issue in fact caused the plaintiffs disease given certain levels of sensitivity and specificity. The base-rateproblem and its effect on reliability of causationopinions are over- looked by judges when scrutinizing the reliability of proffered causationevidence. In this Article, we encouragecourts to consider a Bayes' Law approach to screen out, at an early stage, those claims of injury lacking reliable evidence that an injury was more likely than not caused by exposures to toxic agents. * In private practice and admitted to the bars in Illinois and Missouri; B.S., DePaul University, 1973; M.P.H., University of Michigan School of Public Health, 1977; J.D., Wash- ington University, 1990; Certified Industrial Hygienist (ret.); Member of the Missouri and Illinois bars. ** Principal in Environmental Health at Gradient Corporation, Cambridge, MA; A.B. (summa cum laude), Taylor University, 1964; Ph.D., Harvard University, 1970; M.S., Harvard School of Public Health, 1976; Faculty Member, Amherst College, 1969-76; Faculty Member, Department of Environmental Health, Harvard School of Public Health, 1978-2000. University of MichiganJournal of Law Reform [VOL. 38:4 The goal of our Article is to provide a framework that helps the gatekeeper to screen out toxic tort claims insuffiently substantiatedby the underlyingscientific and medical data, and allow the factflnder to decide only those toxic tort claims for which there is reliableand relvantscientific supportforeach link of the causal chain,frm subject exposure to the in- jury Scientific substantiationof each causal link determines the reliability of an experts opinion that the exposure more liky than not caused the plaintiffs injury. I. INTRODUCTION "Toxic tort" is the label applied to negligence cases in which the plaintiffs allege that exposure to harmful agents, usually chemicals, caused the onset of disease, injury, or even death in others1 As in the norm in negligence cases, a toxic tort plaintiff has the burden of prov- ing that the defendant's negligence caused the injury or illness. The standard of proof is the typical "preponderance of the evidence" stan- dard applied in civil cases.2 In a toxic tort case, the plaintiff must 1. The phrase "toxic tort" is not amenable to precise definition, due to the fact that the term "toxic" is difficult to define. Nevertheless, we offer a practical, perhaps overly sim- ple, definition. In this Article, "toxic tort" refers to personal injury cases in which the plaintiffs claim that exposures to chemicals or radiation caused personal injury. Hearing loss claims based on alleged exposures to noise fit this definition, but generally are not consid- ered toxic tort cases. We discuss in this Article the so-called "trauma cancer" cases which are not strictly toxic torts, but nevertheless display features of generic toxic tort cases, most no- tably a chronic-type injury with a latency period between first exposure and disease onset. See infra note 202. In the remainder of this Article, we use the phrase "plaintiff's injury" to include injuries, diseases, and illnesses incurred not only by the plaintiffs themselves, but also by others, usu- ally decedents or infants. Likewise, the phrase "plaintiff's exposures" encompasses exposures of others for whose injuries the plaintiffs seek recovery. 2. See, e.g., Allison v. McGhan Medical Corp., 184 E3d 1300, 1306 (11 th Cir. 1999) ("The burden of laying the proper foundation for the admission of the expert testimony is on the party offering the expert, and the admissibility must be shown by a preponderance of the evi- dence." (citing Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 592 n.10 (1993))). "In a negligence action, plaintiff has the burden to prove that defendant was negligent by producing evidence that will permit the trier of fact to conclude that it is more likely than not that defen- dant's negligence caused plaintiff's injuries, without any presumption of law one way or another." 65A C.J.S. Negligence § 715 (2000) (citing Vito v. Sargis & Jones, Ltd., 672 A.2d 129 (Md. Ct. Spec. App. 1996), affd, 695 A.2d 191 (Md. 1997), & Anglin v. Kleeman, 665 A.2d 747 (N.H. 1995)). In most jurisdictions, the standard of proof on the issue of causation is that the defendant's conduct "more likely than not" was a substantial factor in producing the plaintiff's injury. See, e.g., Brown v. Parker-Hannifin Corp., 919 E2d 308 (5th Cir. 1990); Sterling v. Velsicol Chem. Corp., 855 F.2d 1188 (6th Cir. 1988); Chaney v. Smithkline Beck- man Corp., 764 F.2d 527 (8th Cir. 1985); Hall v. Baxter Healthcare Corp., 947 F. Supp. 1387 (D. Or. 1996); Reynard v. NEC Corp., 887 F. Supp. 1500 (M.D. Fla. 1995). A party proves a fact by a preponderance of evidence if "the scale tips, however slightly, in favor of the party with the burden of proof." E.g., Ostrowski v. Ad. Mut. Ins. Co., 968 F.2d 171, 187 (2d Cir. 1992) (quoting LEONARD B. SAND ET AL., MODERN FEDERALJURY INSTRUCTIONS 73.01, at 73-74 (1992)). Viewed from the perspective of the party not bearing the burden of proof, the "preponderance standard is ... a tie-breaker dictating that when the evidence on an SUMMER 2005] Toxic Tort Cases establish that exposures to the toxic agent more likely than not caused the injury. Whether the harmful agent caused the injury or disease is the cen- tral issue in most toxic tort cases. Several factors often obscure the causal pathway between the putative exposures and the diagnosed ill- ness. 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 exposure to toxic agents often develop over long periods of time.4 The extended period from exposure to injury obscures the causal relationship, if any, between the exposure and the injuries. In addition, seldom is the injury uniquely traceable to the subject of expo- sure. In most toxic exposure cases, the injury at issue is one that occurs in the general population and has many possible causes, complicating the task of proving a causal link between the suspected toxic agent and the disease.5 To make matters worse, the diagnosis of the disease is sometimes in controversy. Even the allegation of exposure to the agent may be at issue, or at least the duration and intensity of the exposure. Finally, and perhaps most basic, the question of whether the agent is capable of causing the claimed injury is disputable. A circuitous causal chain characterizes "generic" toxic tort cases.6 issue is evenly balanced, the party with the burden of proof loses." United States v.