Statistically Significant Association: Preventing the Misuse of the Bradford Hill Criteria to Prove Causation in Toxic Tort Cases.

• Author: Andrichik, William R.

Volume 86, No. 2

ONE of the highest hurdles for a plaintiff to clear in a toxic tort or product liability case is proof of causation. This is because other key elements of the plaintiff's case, such as product identification, product use and even exposure, typically only require lay testimony. Proof of causation, on the other hand, requires expert testimony--and requires proof "to a reasonable degree of scientific certainty."

The legal system's insistence on the "reasonable degree of scientific certainty" standard is nothing more than a legal fiction created to give courts, counsel and jurors alike the false sense of security that opinions are devoid of scientific guesswork. In reality, however, science is often anything but certain. If it were, experts would offer scientific fact, not opinion.

Lack of certainty is never more evident than in the field of epidemiology, where researchers study the health of human populations. Through a variety of statistical tools, most notably mortality and incidence rates, epidemiologists seek to better understand potential risk factors associated with diseases in such populations. These studies, however, are not designed to achieve certainty in the observed associations, but rather to mitigate factors which could impact those associations, such as chance, bias and other confounding factors (e.g., smoking). Simply put, the less likely that chance, bias and other confounding factors may explain an observed association, the more confident epidemiologists become in their conclusions about the association. As a result, epidemiology is more about confidence in a finding than it is about certainty in a finding.

Having a well-thought-out plan to address an expert's causation opinion--especially in cases where other elements are not (or cannot be) challenged--is vital to avoiding an unfavorable verdict against your client. This article addresses the use of one common methodology used by epidemiologists in toxic tort cases to prove causation: the Bradford Hill criteria.

1. What are the Bradford Hill criteria?

   To reach a conclusion of causation, epidemiologists often consider a set of "criteria" created by Sir Bradford Hill. Sir Bradford Hill was a British physician who studied the association between smoking and lung cancer. 1 To help study associations in epidemiological studies, Sir Bradford Hill developed nine criteria (or "viewpoints" as he described them): 2

   * Strength of association: whether the association is strong and statistically significant.

   * Consistency of the association: whether multiple studies show the same (or substantially similar) results.

   * Specificity of the Association: whether the observed effect has only one known cause.

   * Temporality of the Association: whether the purported cause precedes the observed effect.

   * Biological gradient (aka "Dose-response"): whether a stronger or greater exposure leads to greater amounts of harm.

   * Plausibility: there must be both a rational and theoretical basis for the result.

   * Coherence: whether the cause-effect relationship conflicts with what is already known and whether there...