Abraham Wald, a statistician who worked for the US military during World War II was tasked with analyzing the damage to planes returning from missions and recommending areas for additional armor while optimizing for fuel efficiency and maneuverability.
The military came to Wald with some data they thought might be useful. When American planes came back from engagements over Europe, they were covered in bullet holes. But the damage wasn’t uniformly distributed across the aircraft. There were more bullet holes in the fuselage and none in the engines.
The officers saw an opportunity for efficiency; they thought they could get the same protection with less armor if they concentrated the armor on the places with the greatest need, i.e. where the planes are getting hit the most.
Wald’s insight was simply to ask: “where are the missing holes? The ones that would have been all over the engine casing, if the damage had been spread equally all over the plane? Wald was pretty sure he knew. The missing bullet holes were on the missing planes. The reason planes were coming back with fewer hits to the engine is that planes that got hit in the engine weren’t coming back. Whereas the large number of planes returning to base with a thoroughly Swiss-cheesed fuselage is pretty strong evidence that hits to the fuselage can (and therefore should) be tolerated”. To this, the military analyst had two options for explaining this: either the German bullets just happen to hit every part of the plane but one, or the engine is a point of total vulnerability. Both stories explain the data, but the latter makes a lot more sense.
The armor goes where the bullet holes aren’t. Read more -> https://thedecisionlab.com/biases/survivorship-bias
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