Concidence, probability and asking the right question

Our chief consultant writes:

We’ve all had it happen: some unexpected, unusual occurrence, and someone asks: “What were the odds against that happening?” As an exclamation, an alternative to the pedestrian, “That’s unusual,” this is fine. Language should have flexibility and the freedom of metaphor. This becomes a problem, however, when the unusual event is taken to imply unknown laws of physics or perhaps sinister forces at work.

Actually working out probabilities in any but the simplest cases can be pretty tedious, and we’re not about to get into that here. But it’s easy to make basic mistakes in setting up this kind of question (respected scientists have done so); we present two rules to help keep you out of trouble, even if you’re not going to punch any numbers into your computer.

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