Maths in the dock

By Rebecca Davis 7 October 2011

It is common for forensic experts giving evidence in court to use mathematical formulae to show the probability of someone having committed a crime. But mathematicians are up in arms about the fact that a UK judge has now forbidden the application of statistical analysis to crime trials. By REBECCA DAVIS.

Bayes’ theorem is a mathematical formula which measures how likely something is to be true. In its application to crime, it provides a scientific way to calculate the likelihood of guilt or innocence, or the reliability of evidence. Forensic experts use Bayes’ theorem as a way of quantifying their certainty. In other words, instead of saying “this DNA evidence is probably accurate”, they can say “there is a one-in-three-million chance that this DNA match is wrong”.

But Bayes’ theorem has now been ruled inadmissible by a UK judge. A convicted killer was in the dock last summer appealing against his conviction and among the evidence which had seen him found guilty was a shoeprint from a pair of Nike takkies, found at the crime scene, which seemed to match a pair of Nikes at his house. An expert used Bayes’ theorem to make calculations about the probability of the match being random, which was unlikely: there are 42 million pairs of takkies sold every year, and Nike alone has about 1,200 different sole patterns.

But in this case the judge objected to the fact that the numbers weren’t firm enough. The expert couldn’t say, for instance, exactly how many pairs of these particular Nikes were in the country. As a result, the judge threw out the case and also ruled against the use of statistical analyses in future. Mathematicians say that losing the right to use the theorem will have “shattering” consequences. They are calling on the court of appeal to reconsider the ruling. DM

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Want to watch Richard Poplak’s audition for SA’s Got Talent?

Who doesn’t? Alas, it was removed by the host site for prolific swearing*... Now that we’ve got your attention, we thought we’d take the opportunity to talk to you about the small matter of book burning and freedom of speech.

Since its release, Pieter-Louis Myburgh’s book Gangster State, has sparked numerous fascist-like behavior from certain members of the public (and the State). There have been planned book burnings, disrupted launches and Ace Magashule has openly called him a liar. And just to say thanks, a R10m defamation suit has been lodged against the author.

Pieter-Louis Myburgh is our latest Scorpio Investigative journalist recruit and we’re not going to let him and his crucial book be silenced. When the Cape Town launch was postponed, Maverick Insider stepped in and relocated it to a secure location so that Pieter-Louis’ revelations could be heard by the public. If we’ve learnt one thing over the past ten years it is this: when anyone tries to infringe on our constitutional rights, we have to fight back. Every day, our journalists are uncovering more details and evidence of State Capture and its various reincarnations. The rot is deep and the threats, like this recent one to freedom of speech, are real. You can support the cause by becoming an Insider and help free the speech that can make a difference.

*No video of Richard Poplak auditioning for SA’s Got Talent actually exists. Unless it does and we don’t know about it please send it through.

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