Robin van Schendel retweetledi

During World War II, the U.S. military studied bullet holes on returning planes to decide where to add armor. Most damage appeared on the fuselage, so many thought it should be reinforced.
Mathematician Abraham Wald pointed out a crucial flaw: the data only showed surviving planes. Planes hit in the engine didn’t return at all. This meant the engine was the real weak point.
By reinforcing engines instead, many more planes were saved.
The lesson: good mathematics isn’t just about numbers—it’s about questioning assumptions behind the data.

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