Reporting Exclusionary Results on Complex DNA Evidence, A Case Report Response to 'Uncertainty in Probabilistic Genotyping of Low Template DNA: A Case Study Comparing Strmix™ and Trueallele®' Software
103 Pages Posted: 5 Jun 2023
Date Written: May 18, 2023
Low-level DNA mixtures can be important forensic evidence. In a recent federal case, a California man was accused of drug possession. The defense lawyer requested DNA testing. Cybergenetics’ TrueAllele® system found strong exclusionary likelihood ratios (LR) of a millionth. STRmix™ software found weak LR values near one tenth. The prosecutor retained opposition expert William Thompson, who used the numerical discrepancy to harshly criticize TrueAllele. Who was right?
Fully Bayesian TrueAllele examined all the DNA mixture data to find reproducible match statistics. Probabilistic genotyping (PG) STRmix software only saw about 10% of that data, insufficient for a conclusive answer. Both programs were right, based on different methods and data. Thompson’s case report raised many irrelevant claims, but it ignored the data amounts and betrayed little understanding of PG methods. His report was completely wrong.
The scientific portion of this article corrects Thompson’s mischaracterizations, presenting the PG methods accurately. A STRmix sensitivity study of peak height thresholds shows how different data inputs yield LR values spanning 12 orders of magnitude. The response portion builds on this science to explain why Thompson is mistaken. The article examines 20 of his errors; a supplemental Rebuttal Chart addresses 120 assertions.
Defendants are entitled to meaningful DNA evidence. With low-level mixtures, entering more data and updating more variables can potentially deliver more exculpatory LR information. Thompson advises crime labs to “punt” when they can’t interpret DNA. But using more effective software, they can go for the goal of truth.
Keywords: DNA mixture interpretation, probabilistic genotyping, low-template DNA, Bayesian reasoning, likelihood ratio, complex DNA evidence, TrueAllele system, STRmix software, defendant rights, transposing conditional, error rates
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