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Questioning the Validity of Forensic DNA Match Statistic

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DNA mixtures of two or more people are common. When DNA evidence connects a defendant to a crime, courts require a match statistic. Jurors rely on this match strength to help decide guilt. But the reliability of some match statistics has been questioned.

The Combined Probability of Inclusion (CPI) is a popular match statistic for DNA mixtures. For 15 years, crime laboratories have generated CPI statistics on hundreds of thousands of mixtures. However, a new study shows that CPI behaves more like a random number generator than like a reliable measure of human identification.

The research article “Inclusion probability for DNA mixtures is a subjective one-sided match statistic unrelated to identification information” was published recently. The paper explains why CPI always gives the same “one in a million” answer, instead of providing accurate match information.

“How DNA mixtures are interpreted affects criminal justice and public safety,” says author Dr. Mark Perlin of Cybergenetics. “The inclusion method compares a mixture with a suspect, relying on CPI for statistical support. But if CPI does not accurately convey identification information, its patina of science lends no meaningful support.”

The FBI’s free PopStats software includes a CPI calculator. However, a 2005 NIST study showed 69 crime laboratories reporting a wide range of inaccurate statistics (from ten thousand to hundred trillion, or just “inconclusive”) on the same mixture sample. The FBI has never developmentally validated its CPI software.

The current CPI controversy has shuttered the Washington, DC crime lab, and sparked reconsideration of 24 thousand cases in Texas. Conviction integrity may require re-examination of past CPI mixture cases. Previously reported inculpatory statistics can be irrelevant or unreliable, while an inconclusive result can mask exculpatory evidence.

Cybergenetics develops patented TrueAllele technology that objectively interprets DNA evidence. Seven published peer-reviewed studies havevalidated TrueAllele’s reliability. The system has examined mixture evidence from most states, overcoming admissibility challenges in seven. The company provides computer systems and databases to crime labs, and expert witness services. Cybergenetics offers free TrueAllele screening for calculating accurate match statistics from DNA mixtures.