Likelihood Ratios for Lawyers.

• Author: Valerio, Raymond
• Position: New York

As a NewYork State prosecutor for more than 15 years, it has been my experience that many prosecutors are uncomfortable with scientific evidence. Knowing this, I wrote an article for the WIRE Interdisciplinary Journal. This article highlights the issues prosecutors have faced in NewYork State, but the general principles cross jurisdictional borders. (1)

"I DIDN'T GO TO LAW SCHOOL FOR STATISTICS!"

With the proliferation of probabilistic genotyping software in forensic DNA analysis, prosecutors are faced with the challenge of understanding complex statistical conclusions and their corresponding meanings. Unlike many scientists, lawyers rarely learn statistics in college or law school; statistics are neither a subject on state bar examinations nor a topic in any core continuing legal education course. Therefore, when faced with complicated DNA comparison statistics, prosecutors may unknowingly present misleading--or even incorrect--arguments to the fact-finder. In this primer, I explain how to fairly argue probabilistic genotyping statistics in forensic DNA analysis.

At the conclusion of a trial, before a jury evaluates the evidence, the prosecutor presents a closing argument. In the closing argument, the prosecutor persuasively marshals the evidence and is permitted to make reasonable inferences based on facts adduced during the trial.

To illustrate the significance of appropriate prosecutorial argument, an example helps. On December 11, 2012, Jennifer Ramsaran went missing in Chenango County, New York. After her body was discovered in February 2013, her husband, Ganesh Ramsaran, was charged with murder. During the trial, the prosecution elicited testimony that the defendant filed a missing person report on December 11, the day of his wife's disappearance. Trial testimony highlighted the defendant's whereabouts before and after the missing person report.

Forensic testimony at trial established there was a mixture of DNA on a bloodstain found on Mr. Ramsaran's sweatshirt. The forensic scientist testified that the defendant was the major contributor and that the victim "could not be excluded" as the minor contributor to the bloodstain. Statistical analysis revealed that it was 1.661 quadrillion times more likely that the blood sample from the sweatshirt contained a mixture of the defendant and victim's blood than two randomly selected individuals. The defendant was convicted of murder and sentenced to 25 years to life in prison.

On appeal in 2016, the intermediate appellate court reversed the defendant's conviction and ordered a new trial. The court's analysis centered on the...