Einstein had said - "Not everything that counts can be counted, and not everything that can be counted counts".

 

During World War II, lots of 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 & engineers were called for a short consulting project.  Fighter planes returning from missions were analyzed 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 Army 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 - around the engines.

 

His judgment 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".

 

Einstein had said - "Not everything that counts can be counted, and not everything that can be counted counts".

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