Probability is one of those things we all get wrong... deeply wrong. The good news is we're not the only ones, says John Haigh, a mathematician at the University of Sussex in Brighton, UK, and author of Probability: A very short introduction . "Many pure mathematicians claim that probability has many unreasonable answers."
Take the classic problem of a class of 25 schoolchildren. How likely is it that two of them share the same birthday? The common-sense answer is that it is not implausible, but quite unlikely. Wrong: it's actually just under 57 per cent.
Or the celebrated Monty Hall problem, named after the former host of US television game show Let's Make a Deal. You're playing a game in which there are three doors, one hiding a car, two of them goats (see illustration). You choose one door; the host of the game ...
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