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How Statistics Can Foul the Meaning of DNA Evidence

Posted by timothy on from the persecution-rests dept.

azoblue writes with a piece in New Scientist that might make you rethink the concept of "statistical certainty." As the article puts it, "even when analysts agree that someone could be a match for a piece of DNA evidence, the statistical weight assigned to that match can vary enormously, even by orders of magnitude." Azoblue writes: "For instance, in one man's trial the DNA evidence statistic ranged from 1/95,000 to 1/13, depending on the different weighing methods used by the defense and the prosecution."

3 of 215 comments (clear)

  1. Whaa? by esocid · · Score: 5, Insightful

    In the Smith case, the sample containing another person's DNA showed alleles at seven out of a possible 15 loci, but at four of these loci, the alleles matched those of both the victim and the defendant. "The 1 in 95,000 figure in effect treated these alleles as full-weight evidence that the DNA came from the victim, ignoring the alternative possibility that the allele we saw could have been from the defendant," says Balding. If the opposite position is taken, and these alleles are ignored, you come up with a figure closer to 1 in 13. "It's a question of which loci you consider," he says.

    Since when in the hell do you count common matches as proof that it comes from one person? Some of these labs are doing something very wrong, and I hate to think of both the false positives, and negatives, that came from their "expert" opinions.

    --
    Absolute power corrupts absolutely. indymedia
  2. Re:Hmm... Good by Lunix+Nutcase · · Score: 5, Insightful

    it shouldn't be used to free someone who was justly convicted with other evidence.

    And you know that the other evidence wasn't faulty, how? Police make mistakes, witnesses lie or remember things wrong, etc etc.

    You either believe your justice system is fair or else you scrap the entire thing.

    Or you ditch that false dichotomy and realize that within every system mistakes will be made. There is nothing in fixing past errors that means you throw out the whole system.

    Your alternative would mean that we would have to release every murderer and rapist.

    No, actually it wouldn't.

  3. Re:Lies, Damn Lies, and Statistics! by Kitten+Killer · · Score: 5, Insightful

    You can prove anything with statistics.

    No. You can prove anything with BAD statistics. Unfortunately, most statistics are bad.

    -Scientist Statistician (enough to know that I don't know statistics)