Armour the planes where the bullet holes aren't

In WWII, analysts wanted to reinforce bombers where returning planes showed the most damage. Statistician Abraham Wald flipped it: those planes came back — the fatal hits were wherever the survivors were clean. Every success story you study is a plane that made it home.

— Abraham Wald, A Method of Estimating Plane Vulnerability Based on Damage of Survivors — Statistical Research Group memoranda, 1943; retold in Jordan Ellenberg, 'How Not to Be Wrong' (2014)