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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:

Sampling Theory

Defining a relevant population
Spotting sources of bias and uncertainty
Interpreting confidence intervals

Modeling Profile Probabilities

Testing assumptions
Sensitivity analysis

Probability Theory

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.  


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