* Posts by normanfenton

1 post • joined 5 Oct 2011

Judge cracks down on Bayesian stats dodginess in court


Bayes and the law

This is an interesting article but it is important not to mix up the use of Bayes theorem with poor use of probability. The DNA 1 in a million example cited is simply a classic case of the prosecutor fallacy where the 1 in a million probability of seeing the DNA match evidence in an innocent person is wrongly asumed to the same as the probability of innocence. Bayes theorem actually helps avoid this kind of probabilistic fallacy. The problem with the RvT ruling is that it will have the impact of giving jurors less information than is actually avialable. So, instead of giving some useful probability information about the likelihood of a random match experts will be reduced to vague statements like 'a random match is possible' or 'is unlikely'.

I was interviewed for an article in the Guardian about this case, see:


Here are some links to more detailed information about the issues raised:

About the RvT ruling and the probabilistic issues raised:


The draft proposal for a project to improve the current state of practice:


A report describing common legal fallacies involving probability:


A blog addressing all of these issues:


Norman Fenton

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