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Natural variation in the occurrence of an event can mislead us into thinking that an intervention has been effective when it hasn’t.  For example, if a golfer has had a particularly bad round and she changes her grip on the club, she might shoot a better round the next time.  But is this down to the changed grip, or is it just the natural variation in her score?

This is called “regression to the mean” and is explained very nicely by this poem on the Inkpotamus website.

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