Thursday, 3 February 2011

The Birthday Problem and DNA

Keith Devlin does a variation of the birthday problem involving DNA and finds out how many people must be in a database before there are two whose DNA would match on a 9 locus match (the common judicial standard) as well as pointing out that the Body Mass Index is ....ummm...crap, stupid, not a very good predictor of obesity. 

Good lines.....

"Now, I have no problem with people disagreeing with me. Heavens, I was a mathematics department chair for four years and a dean for eight, and before that a father of two daughters growing up through adolescence to adulthood, so I have had my fair share of disagreements. What bothers me is that some folks seem (1) all too willing to accept something simply because its looks scientific and some august body advocates it, and (2) unable to adjust their intuitions when faced with evidence that they mislead us, as they do on occasion.
It bothers me because I am in the education business, and the primary goal of education is to help people develop the ability to think for themselves and to reach conclusions based on evidence and rational thought.
It bothers me particularly because, as the ancient Greeks recognized, when taught well, mathematics is one of the best mental disciplines to develop analytic thinking skills."

"The fact is, in the era of DNA identification, judges and juries simply cannot avoid getting to grips with the relevant math. Identification hinges on those calculations. There may be no way of avoiding bringing mathematicians into court to explain how the calculations are done. But for that to be effective, those judges and juries need first to learn (and accept) that human intuitions about probabilities are hopelessly unreliable. That can prepare the way, not for mathematical laypersons to learn how to do the calculations themselves - the experts can do that part - rather how to follow the calculations and evaluate the answers. For it is on those answers that justice will ultimately depend."
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