Software Musings

Trial by Ordeal Next?

Posted on: 25/02/2013

It appears that the British judiciary is about to revert to the process of trial by ordeal. Or perhaps declaring that the earth is flat.

Consider the following. While you are not looking, Ethel tosses a coin and writes on a piece of paper whether it came down heads or tails. She puts the piece of paper in a sealed envelope and leaves it on the table. What is the chance that the piece of paper contains the word “heads”? Before you answer 50%, you must know that, in a court of law in Britain that no longer appears to be true.

The last clause of paragraph 35 in a recent judgement ( says: “… and to express the probability of some event having happened in percentage terms is illusory”.

The implication is clear: we cannot assign probabilities to something that has already happened. If you think that the answer to Ethel’s piece of paper is 50% then that’s illusory: as Ethel has already sealed the envelope, the probability of the paper containing the word “heads” is either 0% or 100%. It can’t be 50%.

This is really exciting. The Rev Thomas Bayes died in 1761, leaving us with his wonderful formula P(A|B)P(B) = P(B|A)P(A). Clearly this notable historical event hasn’t reached the upper echelons of the British legal system yet. I wonder how long it will take.

And how many witches we will burn before it gets through. 


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February 2013
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The author of this blog used to be an employee of Nortel. Even when he worked for Nortel the views expressed in the blog did not represent the views of Nortel. Now that he has left, the chances are even smaller that his views match those of Nortel.
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