Slashdot Mirror
British DNA Database Mismatch
nahal writes "DNA evidence is extremely compelling to a jury at trial when trying to convict a suspect. In this article at USA Today, the world's largest DNA crime-solving machine, located in Great Britain, mistakenly matched a suspect to a crime in a 1-in-37 million chance. American experts have called it 'mind blowing'."
Right, 37 million to one is not very big odds when you're doing 245 billion independent tests.
If the probability of a false positive in any individual test is p, then the probability of conducting n tests without getting any false positives is (1-p)^n. As pointed out, this means that if enough tests are done you'll almost certainly convict an innocent person. If you have two crimes with DNA evidence that is only this reliable, then more than likely some innocent person in the UK would test guilty.
Actually, it's worse than this because people don't have independent DNA - they're likely to be distantly related. This makes false positives even more likely.
If there are n people and you want the probabilility that any of them test positive to be less than x then you need
1 - (1-p)^n < x, which is nearly the same as 1 - p*n < x. So to be fairly sure that nobody in the world falsely tests positive you need p to be less than about 1 in 80 billion.
perl -e 'fork||print for split//,"hahahaha"'
The probability of a false positive match approaches 1 as the number of samples approaches oo.
P(false positive) = 1 - P(no false positives)
= 1 - (P(correct answer))^n
= 1 - (1-p)^n
-> 1 as n -> oo.
This is ignoring the probability of a false negative; this is very low since only one person can commit a crime!
perl -e 'fork||print for split//,"hahahaha"'
No, the original poster was right. The chance of a false match on the file is
1 - (1 - 1/37million) ^ 660,000
which is nearly the same as
660,000 / 37million = 1/56.
perl -e 'fork||print for split//,"hahahaha"'
This is so basic, I can't even believe it! I can't believe peoples lives are decided on such a weak mathematical basis!
If the chance of a match between two random DNA samples is 1/37.10^6, and they have 660000 samples in their database, then the likelihood -- assuming their system does'nt give false positives, which I doubt -- of a database match is ... 1.78% !!! We don't know how much DNA tests they make each year, but it's porbably well over a thousand, wich leads to over 10 false positives a year!
Americans find that "mind blowing"? Minboggling stupidity, if you ask me