DNA Evidence: Case Study in Probabilities

This educational module on DNA evidence presents trial testimony, exhibits, and opinions in a case in which federal courts at every level discerned “inaccuracies” in the testimony of a leading expert about probabilities associated with the DNA evidence. By embedding these legal materials in background explanations, critical questions, and short problems, the module supports self-study and class discussions that together can elucidate key principles in scientific reasoning and quantitative analysis and that can help students avoid a classic and common misunderstanding about conditional probabilities.

The module includes information and exercises relating to: (1) Sampling theory (including defining a relevant population, spotting sources of bias and uncertainty, interpreting confidence intervals); (2) Modeling profile probabilities (including testing assumptions, sensitivity analysis); and (3) Probability theory (including the concept of mathematical probability, probabilities of events such as a DNA match to a randomly selected individual, independent and dependent events, conditional probability and the transposed conditional, likelihood ratios for pairs of hypotheses as a measure of the probative value of DNA matches and other scientific test results, Bayes’ rule for deducing a posterior probability from a likelihood ratio and a prior probability).

The module was constructed with law students in mind, but the materials and questions are not intended for the traditional legal exercise of extracting the holdings of the courts and assessing the legal arguments for and against them. Rather, the goal is to give students in all fields the analytical tools needed to evaluate the presentation of the scientific evidence in the case. The topics of sampling, weight of evidence, and updating probabilities in response to new scientific evidence are of obvious importance in the domains of medicine, public policy, and business as well as law.

The most efficient way to cover the materials in class is with a minimum of lecturing and a maximum of class discussion and exercises such as a simulated cross-examination of the expert witness in the case. An instructor's manual offers detailed suggestions. In law schools, the module could be employed in advanced courses or seminars that cover expert evidence, scientific evidence, law and science, or law and genetics. The case materials also could be the basis for a component of a trial advocacy course involving expert witnesses.

This module is one of 9 modules developed by the Committee on Preparing the Next Generation of Policy Makers for Science-Based Decisions, an ad hoc committee under the auspices of the National Academies of Sciences, Engineering, and Medicine’s Committee on Science, Technology, and Law. Access all modules.