During WWII, the Navy tried to determine where they needed to armor their aircraft to ensure they came back home. They ran an analysis of where planes had been shot up, and came up with the diagram above. The meaning, so they believed, was plain..
Obviously the places that needed to be up-armored are the wingtips, the central body, and the elevators. That’s where the planes were all getting shot up.
Abraham Wald, a statistician, disagreed. He thought they should better armor the nose area, engines, and mid-body. Which was crazy, of course. That’s not where the planes were getting shot.
Except Mr. Wald realized what the others didn’t. The planes were getting shot there too, but they weren’t making it home.
What the Navy thought it had done was analyze where aircraft were suffering the most damage. What they had actually done was analyze where aircraft could suffer the most damage without catastrophic failure. All of the places that weren’t hit? Those planes had been shot there and crashed. They weren’t looking at the whole sample set, only the survivors.