Those Magnificent Men in Their Flying Machines

Some people are just plain clever. 

Abraham Wald is one of them. He was a Hungarian mathematician who helped out the British Air Ministry in WWII after they comissioned a study to help them provide better protection for the bombers which were flying over enemy territory.  This involved mapping the bullet holes on returning planes, to find out where they took the most damage from enemy fire.

Planes were inspected, the analysis was done and it became apparent that most of the damage was on the extremities  - rather than to the main fusulage or engine areas.   So the Air Ministry prepared to add extra reinforcement to these areas of the planes, to offer pilots greater protection.

It took Abraham Wald to point out that they were missing the obvious: it was only the planes which came back which could be mapped.  The other planes, the ones which didn't survive, had also been hit by bullets; and the fact that no damage was recorded in certain areas suggested that these were the areas where a single bullet could do the most damage.  It would be a much better idea, he argued, to add extra armour to the parts of the planes which the data suggested were hit the least, becuase the planes which had been hit there were were the ones which didn't make it back.

There's a better explanation here; it's an old story, but one which popped up on Twitter today, via the brilliant-as-always Graham Linehan.