How may survivors lead us to false conclusions? What threats are hidden under our natural tendency to concentrate on the people or things that made it through?

During World War II, statistician Abraham Wald was asked to catalog the location of bullet holes on returning Allied aircraft and determine the best places to reinforce the planes with armor. Wald developed the diagram, shading the areas where bullets pierced the returning airplanes. Shaded areas were mostly the nose, wings, and fuselage.

Naturally, Wald’s supervisors concluded that the nose, wings, and fuselage were covered in damage and therefore needed more armor. Instead, Wald suggested that the shaded areas did not need reinforcement and the Allies should armor the areas untouched by bullets. Wald reasoned that the aircraft returned home because neither the cockpit nor the tail sustained damage. He recognized survivorship bias in the sample; the Allies were only sampling planes that completed their missions and returned home. Abraham Wald saw what everyone else could not: the aircraft that didn’t make it home.

Source: The National WWII Museum Blog

Photos: Abraham Wald. University of St Andrews, Scotland

American B-17 bombers over Germany. The National WWII Museum

Wald's diagram of bullet damage. The National WWII Museum