Brilliant insight courtesy of Tim Harford
Tom Salinsky writes...
In 1943, the American statistician Abraham Wald was asked to advise the US air force on how to reinforce their planes. Only a limited weight of armour plating was feasible, and the proposal on the table was to reinforce the wings, the centre of the fuselage, and the tail. Why? Because bombers were returning from missions riddled with bullet holes in those areas.
Wald explained that this would be a mistake. What the air force had discovered was that when planes were hit in the wings, tail or central fuselage, they made it home. Where, asked Wald, were the planes that had been hit in other areas? They never returned. Wald suggested reinforcing the planes wherever the surviving planes had been unscathed instead.
The full article is here.