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Good answers to bad questions about DNA match statistics

Perlin, M.W. Good answers to bad questions about DNA match statistics. Forensic Magazine, 16(1):32-34, 2019.




Direct examination is over, and the DNA evidence has been presented. You’ve addressed chain-of-custody, crime lab processing, data analysis, and mixture interpretation. The match statistic is a trillion. You’re almost done.

And then opposing trial counsel asks about simple arithmetic. The questions don’t seem relevant to your DNA match statistic. But the jury wakes up, and is now paying close attention. They wonder, is the defendant’s DNA really in the evidence?

This DNA mixture scenario has played out in courtrooms for decades. The match statistic captures uncertainty, so an adversary tries to evoke doubt. His math is wrong, his arguments irrelevant. But twisting probability can confuse the jury, and affect the outcome.


M.W. Perlin, "Distorting DNA evidence: methods of math distraction", American Academy of Forensic Sciences 70th Annual Meeting, Seattle, WA, 22-Feb-2018.