During WWII, the RAF had to decide where on their planes to put the armour plating. They could not plate everywhere due to weight considerations. They were goin

During WWII, the RAF had to decide where on their planes to put the armour plating. They could not plate everywhere due to weight considerations. They were going to put it on places where returning aircraft were damaged, but Patrick Blackett (http://en.wikipedia.org/wiki/Patrick_Blackett,_Baron_Blackett http://en.wikipedia.org/wiki/Patrick_Blackett,_Baron_Blacket...) pointed out that this was a biased sample. His recommendation was to put the plating were returning aircraft were not damaged, as this is probably where the aircraft which did not return were hit.