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Bryan Krahn first tells this story in an article titled, “Survivorship Bias in Fitness,” When planes began returning to base riddled with bullet holes during WWII, the US Army Air Forces (USAAF) knew it had a problem. How could they shore up the planes to better protect the crew? The brightest minds at the USAAF immediately began looking for ways to reinforce the damaged areas. They studied the areas riddled with bullet holes, particularly the wings and tail. That is until Abraham Wald, a statistician and mathematician, made a key observation — they were only looking at the damage on returning planes. They hadn’t factored in damage inflicted on planes that failed to return. He proposed bolstering the armor where there were no hits, like around the engines and the cockpit. Unlike the tail, body, and wings, these areas were extremely vulnerable. Planes hit there never made it back to base to have their damage charted out.
This story illustrates the concept of survivorship bias. Survivorship bias is the logical error of concentrating on the people or things that made it past some selection process and overlooking those who did not, typically because of their lack of visibility.