You are serving on a jury in the trial of a man accused of murdering his wife. It emerges that the accused had regularly beaten his wife—but his highly paid defence team presents statistics supposedly showing that only 1 in 1000 wife beaters go on to kill their wives. The conclusion seems obvious: you should not make much of the wife-beating evidence. Wrong. You have just fallen into a trap that awaits anyone who relies on "common sense" to understand evidence based on probabilities. For that 1 in 1000 figure is a red herring. You should be focusing not on the odds of wife beaters going on to kill their wives, but the odds that a wife beater whose wife has been murdered is responsible for her death.
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