Why politicians looking for blanket powers of surveillance, screening or anything else need to learn some maths.

Yesterday the UK suggested that an unnamed internet company could have prevented the murder of a soldier by taking action based on a message sent by the murderer.

It’s assumed that the company in question was Facebook.

The problem is that the maths tells us that this is simply wrong. It couldn’t have, and the reason why takes us to a graveyard near Old Street.

Buried in Bunhill Fields is Thomas Bayes, a non-conformist preacher and part time statistician who died in 1761. He devised a theorem (now known as Bayes Theorem) that helps us to understand the real probability of infrequent events, base on what are called prior probabilities. And (thank God) events like this murder are infrequent.

For the sake of argument let’s imagine that Facebook can devise a technical way to scan through messages and posts and determine if the content is linked to a terrorist…

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