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Sunday, July 22, 2007

Demystifying DNA (Part II)

A while back I posted a critique of DNA evidence based in part on my own understanding of the science and also on an intriguing article of Keith Devlin's. Usually, I get a comment here or their. Either "agree" or "disagree," but rarely anything as informative as the one I recently received to this post.
Two points: Devlin has disavowed the column that you quoted extensively. In a later column he makes clear that he knows better, and claims that the earlier column was meant to represent the kind of misunderstanding that a lay person might have. My web page is one of many analyses showing that there is, contrary to the apparent (but disavowed) message of Devlin's early column, nothing "scary" about the observed data on partial DNA matches in the Arizona CODIS database or anywhere else.

Second, while 15 quadrillion may be an unrealistic number in various respects, your criticism that it is contrived because it represents inter-ethnic matching chances is not correct. It represents the typical matching chance for two randomly selected unrelated persons of the same ethnicity. If we drop the artificial "unrelated", yes the chance increases but not notably unless the randomly selected people happen to be close relatives (nephew, son, brother).

Charles Brenner, PhD
Forensic mathematics

I recommend reading through his links. He makes a pretty thorough rebuttal of Devlin's (and my) arguments. Although, I would just like to say that my primary point, that DNA is statistical evidence just like all other evidence, still stands. It should not be used by itself to convict. Only in the presence of other evidence do we see anything remotely close to "beyond a reasonable doubt.

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