Challenging expert witness opinions on future disease outcomes: what attorneys need to know about the epidemiological concept of relative risk

Most attorneys are likely familiar with the notion that “correlation does not equal causation.” Yet, this critical distinction can often get lost once math and statistics get involved. In litigation where injury causation is an issue, expert witnesses may cite studies that show a group of people exposed to a particular risk factor (such as a toxin, a traumatic injury, or something else) are more likely to experience some outcome compared to people who have not been exposed to that particular risk factor. For example, it is well-established that people who smoke cigarettes are more likely to develop lung cancer compared to people who do not smoke cigarettes.

However, just because people exposed to a risk factor are more likely to experience an outcome does not mean that the exposure causes the outcome, especially when the outcome is also possible in the absence of that particular risk factor. Lung cancer can sometimes occur in people who have never smoked. If that is the case, how do we know that smoking cigarettes causes lung cancer, instead of being merely correlated with lung cancer? As discussed in our previous blog post on the Bradford Hill Criteria, while there are multiple factors to consider in determining whether an observed correlation can be deemed causative, the “strength of the association” between the risk and the disease is one of them.

Enter relative risk. Relative risk is the incidence rate of a disease in a group of people who share a risk factor (such as a behavior, an exposure, a genetic variant, an injury, etc.) compared to the incident rate of the disease in people who have not been exposed to that risk factor. The incidence rate of a diease is percentage of a study cohort that develops a disease within a specific time period. As explained by the Federal Judicial Center’s Reference Manual on Scientific Evidence (3d ed. 2011), “if 10% of all people exposed to a chemical develop a disease, compared to 5% of people who are not exposed, then the disease occurs twice as frequently among the exposed people,” equivalent to a relative risk of 2. If the relative risk is 1.0, there is no association between the agent and the disease. If the relative risk is greater than 1.0, “there is a positive association between exposure to the agent and the disease, which could be causal.” Id. at 567.

Daubert v. Merrell Dow Pharmaceuticals provides a helpful explanation of relative risk within the context of the Daubert factors. The issue in that case was whether the plaintiffs’ exposure to the drug Bendectin in utero caused the plaintiffs to develop limb shortening birth defects. After the Supreme Court articulated its new Daubert factors and remanded the case to the Ninth Circuit, that court explained the role that relative risk plays in the admissibility of an expert opinion that exposure to a particular risk factor caused a disease. Evaluating the plaintiffs’ experts’ causation opinions under the new Daubert factors, the Ninth Circuit explained:

To evaluate the relationship between Bendectin and limb reduction defects, an epidemiologist would take a sample of the population and compare the frequency of birth defects in children whose mothers took Bendectin with the frequency of defects in children whose mothers did not. The ratio derived from this comparison would be an estimate of the "relative risk" associated with Bendectin. For an epidemiological study to show causation under a preponderance standard, "the relative risk of limb reduction defects arising from the epidemiological data . . .

will, at a minimum, have to exceed '2'." That is, the study must show that children whose mothers took Bendectin are more than twice as likely to develop limb reduction birth defects as children whose mothers did not. While plaintiffs' epidemiologists make vague assertions that there is a statistically significant relationship between Bendectin and birth defects, none states that the relative risk is greater than two. These studies thus would not be helpful, and indeed would only serve to confuse the jury, if offered to prove rather than refute causation. A relative risk of less than two may suggest teratogenicity, but it actually tends to disprove legal causation as it shows that Bendectin does not double the likelihood of birth defects.

Daubert v. Merrell Dow Pharmaceuticals, 43 F.3d 1311, 1321 (9th Cir.1995). (internal citations omitted) (emphasis added).

Thus, for an epidemiological opinion to show causation under the Federal courts’ preponderance standard, the opinion must be based on data showing that exposure to a particular risk factor increases the relative risk of developing the disease outcome by a factor of two or greater. Studies that show people are slightly more likely to develop a disease when exposed to a risk factor would not satisfy this standard.

It is important to understand that relative risk relates to only one component of injury/disease causation analysis—the concept of general causation, which is a question of whether a risk factor is capable of causing a particular disease. Even if an expert can successfully establish general causation by showing that a risk factor more than doubles the relative risk of developing a disease, the expert must still establish specific causation by showing that the patient’s exposure to the risk factor actually did cause the disease.

In traumatic brain injury litigation, in support of claims for future medical expenses, experts commonly assert that a person who has sustained a TBI will develop a variety of conditions in the future as a result of the TBI. Common examples include neuropsychiatric conditions such as Alzheimer’s disease. However, even if the expert testifies that the research shows increased incidence of certain diseases in people that have sustained a TBI, it does not necessarily mean that the expert can reliably testify, consistent with the Daubert principles, that a TBI can cause (or will cause) the future outcome. Understanding how relative risk works can help attorneys critically evaluate, and if necessary, challenge, an expert’s opinion on causation or prognosis in such cases.