Counter intuitive.....................................?

"Look at where the bullet holes are and put extra armor every place else."

During World War II, the statistician Abraham Wald tried to determine where one should add extra armor to airplanes.  Based on the pattern of bullet holes in returning airplanes, he suggested that the parts not hit should be protected with extra armor.  How could he reach such a conclusion?  Because he also considered planes that didn't return.  Assume that all planes had been hit more or less uniformly.  Some planes hit in marked areas were still able to return.  This means that planes that didn't return were most likely hit somewhere else - in unmarked places.  Those were the places that needed more armor.
-Peter Bevelin, as extracted from Seeking Wisdom:  From Darwin to Munger