The Interpretation of DNA Evidence A Case Study in Probabilities An Educational Module
David H. Kaye Associate Dean for Research and Distinguished Professor The Pennsylvania State University (Penn State Law)
DNA profiling has been profoundly disruptive of business as usual in forensic science and criminal justice. It spawned an innocence movement dedicated to correcting and preventing wrongful convictions, and it dramatically changed the investigation and prosecution of sexual assault cases. It sparked intense controversy among statisticians and population geneticists and ultimately stimulated efforts toward overtly probabilistic forensic identification for more complicated pattern and impression evidence.
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. More specifically, the module includes information and exercises relating to the following scientific and statistical principles and skills:
Defining a relevant population Spotting sources of bias and uncertainty Interpreting confidence intervals
Modeling Profile Probabilities
Testing assumptions Sensitivity analysis
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.
TO REQUEST A COPY OF THE INSTRUCTORS' GUIDE FOR THIS MODULE, EMAIL US AT email@example.com.
The copyright in the linked module is owned by the author(s) of the module, and may be used subject to the terms of the Creative Commons Attribution-NonCommercial 4.0 International Public License. By clicking on the link and using or further adapting the educational module, you agree to comply with the terms of the license. The educational module is solely the product of the author(s) and others that have added modifications and is not necessarily endorsed or adopted by the National Academy of Sciences, Engineering, and Medicine or the sponsors of this activity.