The planes that came back were a biased sample
During World War II, analysts examining returning bombers mapped the bullet holes they could see. The pattern looked like a guide to where aircraft were most often hit—and, therefore, where added armor might help. But the records included only planes that had survived their missions.
That distinction changes what the holes mean. The maps showed damage on aircraft that returned, not every place enemy fire had struck across the fleet. A plane hit in a relatively survivable area might make it home with visible damage. A plane hit in a vital area, such as an engine, might never appear in the sample at all.
What the missing holes revealed
The areas with the fewest holes on surviving planes could be the most dangerous to hit. If strikes there often caused a bomber to be lost, those planes—and their damage—would be absent from the analysis. In statistical terms, the researchers were studying survivorship: the visible survivors were not representative of all the aircraft that had been attacked.
The practical lesson was to reinforce vulnerable areas that looked unusually unmarked on the returning planes, rather than simply adding armor wherever holes were most common. Armor added weight, so placing it carefully mattered: extra protection had to reduce the risk of a fatal hit without burdening the aircraft more than necessary.
A blank patch on the map was not evidence that a spot was safe. It could be the mark left by the planes that never came back.
