During WWII, Allied bomber losses were high, so high that the British Air Ministry undertook a rigorous analysis in hopes of finding a solution. Their engineers set out to eyeball every bomber they could, gathering data on each bullet hole. After analyzing the results, engineers decided to reinforce the areas that had the highest concentrations of holes with armor plating.

It didn’t work.

Perplexed, the engineers assumed that the extra plating had made the planes too heavy, and that the difficulty in handling the planes was offsetting the protection of the armor plating.

Enter Abraham Wald.

Wald, a mathematician, suggested that they simply put extra armor plating where the bullet holes weren’t. The idea was simple: if the planes are returning with bullet holes, obviously those areas can be struck without causing the planes to crash. The planes that weren’t returning, Wald theorized, are the ones that are getting hit in different areas.

The engineers’ error was so significant, statisticians decided to name it:

Survivorship Bias
(the tendency to include only successes in statistical analysis).

Any time you only examine just the successes, you will skew the results.
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