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During World War II, numerous fighter planes were getting hit by anti-aircraft guns. Air Force officers wanted to add some protective armor/shield to the planes. The question was “where”?The planes could only support few more kilos of weight. A group of mathematicians and engineers were called for a short consulting project. Fighter planes returning from missions were analysed for bullet holes per square foot. They found 1.93 bullet holes/sq. foot near the tail of the planes whereas only 1.11 bullet holes/sq. foot close to the engine.The Air Force officers thought that since the tail portion had the greatest density of bullets, that would be the logical location for putting an anti-bullet shield. 

A mathematician named Abraham Wald said exactly the opposite; more protection is needed where the bullet holes aren’t – that is -around the engines.His judgement surprised everyone.  He said “We are counting the planes that returned from a mission. Planes with lots of bullet holes in the engine did not return at all.”

If you go to the recovery room at the hospital, you’ll see a lot more people with bullet holes in their legs than people with bullet holes in their chests.  That’s not because people don’t get shot in the chest; it’s because the people who get shot in the chest don’t recover.

Remember the words of Einstein – “Not everything that counts can be counted, and not everything that can be counted, counts.”

Excerpted from – “How Not To Be Wrong” by Jordan Ellenberg