NC attorneys may be aware that software programs are being adopted by crime laboratories to assist with interpreting complex DNA mixtures. The NC State Crime Laboratory is working on validating a procedure for the use of STRmix probabilistic genotyping software, which will likely go online in the coming months.
It is crucial that attorneys understand exactly what DNA testing is being used in a case. In a law review article examining the value of probabilistic genotyping and the likelihood ratios that probabilistic genotyping software produces, Public Defender and NY Legal Aid Society Staff Attorney Bess Stiffelman argues that the results generated by this methodology can be misunderstood by jurors and attorneys.
The nature of probabilistic genotyping, Stiffelman argues, raises problems that the article discusses in detail. Unlike the comparison of a single-source DNA sample to a suspect’s DNA, probabilistic genotyping seeks to interpret complex mixtures and fill in gaps of an incomplete and damaged DNA sample to discover the likelihood that a suspect produced it. While single-source matching produces a Random Match Probability which compares a sample to the probability for all outcomes, probabilistic genotyping produces a likelihood ratio calculating which of two pre-established hypotheses is more likely. It is important for attorneys to consider what hypotheses are being compared.
In an illustrative analogy, Stiffelman asks which is more likely when confronted with a destroyed pillow sitting next to your dog: that the pillow’s seams spontaneously fell apart or that a burglar shredded it while you were out? After gathering facts, you find it 10,000 times more likely that the pillow fell apart. While the likelihood ratio is greatly in favor of spontaneous destruction, it utterly fails to take into account other possible hypotheses (bad dog) that may be more likely. Probabilistic genotyping only looks at the likelihood of two predetermined hypotheses. If not properly explained, the ratios produced by the analysis can easily mislead jurors and attorneys who lack understanding of what is being compared and a compelling statistic may be produced that potentially overlooks an even more likely hypothesis.
In the article, Stiffelman also examines probabilistic genotyping in light of the constitutional right to the presumption of innocence in a criminal trial, current case law, and the effects of an incorrectly interpreted likelihood ratio. In general, the article provides an excellent starting point for attorneys looking to gain a better understanding of this method, what it offers, and some of its potential pitfalls if it is misinterpreted.
The National Institute of Justice’s Forensic Technology Center of Excellence offered a webinar series on probabilistic genotyping in 2019. The archived programs are available for free viewing here.