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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."

8 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:Damn Lies and Statistics! by finarfinjge · · Score: 4, Insightful

    The trouble that this paper and many others illustrate is the HUGE ignorance of proper statistical methods in the scientific community. Such things like a students T test are - statistically speaking - simple. Yet they are often beyond many in the science community. Thus, there is a tendency for misuse of technique, which in turn leads to divergent interpretations of what a data set means. The legal profession is even worse, as they don't care about the laws of mathematics. In a court, you are not required to answer to a professor of mathematics, hence you can assert anything. If your opponent doesn't have the necessary skill or knowledge to call BS on what you say, you can win an argument with a completely baseless assertion. Take an example. A man is fired for missing work on a Monday. The company's lawyer states "Fully 40% of this employee's absenteeism occurs on Mondays and Fridays. It is appalling that this weekend extending behaviour continues, and we must do something about it". The mathematically challenged lawyer for the poor sap can't see the issue with this and lets it stand.

    JE (always wanted to use that example. May have the justification a bit!)

  3. Re:1/13 by JW+CS · · Score: 4, Insightful

    Sadly, your average American juror might find that logic compelling.

  4. 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.

  5. 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)

  6. Re:It's fine for saying "it's somebody else". by sjames · · Score: 4, Insightful

    Simply, DNA evidence is by nature exclusionary. The scientifically correct result of a DNA test is excluded or not-excluded.

  7. Re:People don't understand statistics by terrymr · · Score: 4, Insightful

    DNA testing probabilities go something like this ....
    we found say 5 markers that match the defendant and the sample. (I picked a small number to make the example shorter)
    each of those has the following probabilities of occuring in a random person :

    1) 1 in 1000
    2) 1 in 10
    3) 1 in 10000
    4) 1 in 7
    5) 1 in 100

    so we multiply all those together and get a probability of mismatch of : 1 in 7,000,000,000

    I even told a guy at the state crimelab that was stupid - not that he cared.

  8. Re:People don't understand statistics by HungryHobo · · Score: 4, Insightful

    If the only thing that pointed you at him was that search of the database then it tells you almost nothing about how likely he is to be guilty on it's own.

    If you find a suspect be searching through a database of a million people with a test that has a 1 in a million chance of making a false positive and no other evidence exists then the chances of that match should not be used in any way to establish guilt in court.
    But then lawyers don't care about using stats correctly.

    If however you find someone, they have a knife with the victims blood on it and they have a motive and you compare their DNA to the DNA found at the scene then that same test with a 1 in a million chance of a false positive is a perfectly valid piece of data to submit in court.