via Arthur Robson:

While appeals often unmask shaky evidence, this was different. This time, a mathematical formula was thrown out of court. The footwear expert made what the judge believed were poor calculations about the likelihood of the match, compounded by a bad explanation of how he reached his opinion. The conviction was quashed.

And the judge ruled that Bayes’ law for conditional probabilities could not be used in court. Statisticians, Mathematicians, and prosecutors are worried that justice will suffer as a result. The statistical evidence centered around the likelihood of a coincidental match of shoeprint with shoes owned by the Defendant.

In the shoeprint murder case, for example, it meant figuring out the chance that the print at the crime scene came from the same pair of Nike trainers as those found at the suspect’s house, given how common those kinds of shoes are, the size of the shoe, how the sole had been worn down and any damage to it. Between 1996 and 2006, for example, Nike distributed 786,000 pairs of trainers. This might suggest a match doesn’t mean very much. But if you take into account that there are 1,200 different sole patterns of Nike trainers and around 42 million pairs of sports shoes sold every year, a matching pair becomes more significant.

Now if I can prove to jurors that there was one shoe in the basement and another shoe upstairs, then probably I can legitimately claim to have proven that the total number of shoes is two because the laws of arithmetic should be binding on the jurors deductions. And if there is a chance that a juror comes to some different conclusion then it would make sense for an expert witness, or the judge even, tell the juror that he is making a mistake. Indeed a courtroom demonstration could prove the juror wrong.

But do the “laws” of probability have the same status? If I can prove to the juror that his prior should attach probability p to A and probability q to [A and B], and if the evidence proves that A is true, should he then be required to attach probability q/p to B? Suppose for example that a juror disagreed with this conclusion. Could he be proven wrong? A courtroom demonstration could show something about relative frequencies, but the juror could dispute that these have anything to do with probabilities.

It appears though that the judge’s ruling in this case was not on the basis of bayesian/frequentist philosophy, but rather about the validity of a Bayesian prescription when the prior itself is subjective.

The judge complained that he couldn’t say exactly how many of one particular type of Nike trainer there are in the country. National sales figures for sports shoes are just rough estimates.

And so he decided that Bayes’ theorem shouldn’t again be used unless the underlying statistics are “firm”. The decision could affect drug traces and fibre-matching from clothes, as well as footwear evidence, although not DNA.

This is a reasonable judgment even if the court upholds Bayesian logic per se. Because the prior probability of a second pair of matching shoes can be deduced from the sales figures only under some assumptions about the distribution of shoes with various tread patterns. The expert witnesses probably assumed that the accused and a hypothetical third-party murderer were randomly assigned tread patterns on their Nikes and that these assignments were independent. But if the two live in the same town and shop at the same shoe store and if that store sold shoes with the same tread pattern, then that assumption would significantly understate the probability of a match.

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November 20, 2011 at 9:04 pm

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September 15, 2013 at 10:45 pm

Prosecution Bias | Cheap Talk[…] are conditional probabilities so rarely used in court, and sometimes even prohibited? Here’s one more good reason: prosecution […]