During World War II, a mathematician named Abraham Wald worked on a problem – identifying where to add armor to planes based on the aircraft that returned from missions and their bullet puncture patterns. The obvious and accepted thought was that the bullets represented the problem areas for the planes. Wald pointed out that the problem areas weren’t actually these areas, because these planes survived. He found that the missing planes had unknown data, indicating other problem areas existed. In fact, the pattern for the surviving planes showed the areas that weren’t problematic.
By McGeddon - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=53081927