Abraham Wald was a peculiar Jewish mathematician born in Europe who migrated to the USA to escape from the Nazis. Wald applied his brains to a seemingly simple task: evaluating airplanes’ vulnerability. To do so, he observed the aircrafts full of holes that came back from the front. Quite simple, right? One should reinforce the more damaged patches to give them better chances of a successful return.
However, after elaborating sophisticated analysis techniques, his unique recommendation may sound insane. Wald suggested that the patches that had not been damaged were actually the most vulnerable, and they were the ones where armour should be added first. Something like wearing a band-aid where there is no wound. What for?
The answer is in the aircraft he analysed – they were the ones that had actually returned from the front. Wald’s insight was taking took into consideration that the airplanes that had returned were the ones that had made it through all the misadventures. The holes were a tell-tale sign of the strongest spots, the ones which could resist the mishaps, not the weakest areas. It was the other way round: the pristine spots pinpointed the places that could not be hit, otherwise the planes would have been lost in combat and would never be analysed by him back at home. The ones that did return with intact weakest areas had been lucky.