The Mind Mechanic

Survivorship bias: the damage you observe is conditional on survival. In 1943, the Statistical Research Group (SRG) at Columbia University was tasked by the United States military with determining where to add armor to combat aircraft. Armor plating adds significant weight, and every extra kilogram reduces speed, range, or payload. Because armor could only be applied selectively, where would it provide the greatest protective benefit? Ground crews surveyed returning bombers and mapped the distribution of bullet holes. Hits were visibly clustered on the fuselage, wings, and empennage. A naive reading of the returning-aircraft data would suggest reinforcing the most heavily damaged areas. Abraham Wald’s analysis showed why that inference could be backwards. Wald recognized that the observed damage pattern was conditional on survival. Returning aircraft showed relatively fewer hits in some areas—such as the engine compartments—but the damage distribution on aircraft that had be...