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pop article and still too forgiving, but this is not a bad start: https://freakonomics.com/2008/08/are-the-fbis-probabilities-about-dna-matches-crazy/ https://f

pop article and still too forgiving, but this is not a bad start: https://freakonomics.com/2008/08/are-the-fbis-probabilities-about-dna-matches-crazy/ https://freakonomics.com/2008/08/are-the-fbis-probabilities-... (it's still excessively charitable about the odds because the genetic factors are not really perfectly independent and even a tiny bit of interdependence wreaks havoc with large probability calculations) The link you cite with "1 in billions" odds is exactly the sort of misrepresentation I'm talking about. Combine smaller-than-pretended probabilities with big database trawling and it becomes a bit like the "birthday problem": it provides really good odds of finding someone to pin the crime on, but very little confidence that it was really them.

asdff 4y ago
The birthday problem is different because there are 365 possibilities and every individual can only have 1 out of 365 states. For DNA there are over 300 million variants in the human genome and you can have different combinations of these variants. As far as matching 9 of 13 loci, thats not surprising to me nor concerning. If you left a sample at a crime scene it would match all 13 loci. Today the FBI uses at least 20 loci. On top of that, merely being a dna match does not mean you are going to jail. You need corroborating evidence just like how finger prints are used. The odds of a false positive that 1) is a dna match and 2) was in the right place at the right time and lacks and alibi and has a potential motive for the crime must be a trillion to one.
- yetanotherloser 4y ago
  Birthday problem analogy comes up when trawling databases, not in single matches. Even in single matches you need to be careful not to apply the maths of combining independent probabilities on things that aren't really independent. As soon as someone starts quoting one-in-billions it is reasonable to fear that they might be trying to pull the "prosecutor's fallacy". Was the article I linked helpful?

asdff 4y ago

The birthday problem is different because there are 365 possibilities and every individual can only have 1 out of 365 states. For DNA there are over 300 million variants in the human genome and you can have different combinations of these variants. As far as matching 9 of 13 loci, thats not surprising to me nor concerning. If you left a sample at a crime scene it would match all 13 loci. Today the FBI uses at least 20 loci. On top of that, merely being a dna match does not mean you are going to jail. You need corroborating evidence just like how finger prints are used. The odds of a false positive that 1) is a dna match and 2) was in the right place at the right time and lacks and alibi and has a potential motive for the crime must be a trillion to one.

yetanotherloser 4y ago
Birthday problem analogy comes up when trawling databases, not in single matches. Even in single matches you need to be careful not to apply the maths of combining independent probabilities on things that aren't really independent. As soon as someone starts quoting one-in-billions it is reasonable to fear that they might be trying to pull the "prosecutor's fallacy". Was the article I linked helpful?