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One of the most famous figures to illustrate this skill is the mathematician Abraham Wald (Mangel and Samaniego 1984). During World War II, he was asked to help the Royal Air Force find the areas on their planes that were most often hit by bullets so they could cover them with more armour. But instead of counting the bullet holes on the returned planes, he recommended armouring the spots where none of the planes had taken any hits. The RAF forgot to take into account what was not there to see: All the planes that didn't make it back.