During the Second World War, the U.S. military studied bombers returning from missions and mapped where they were riddled with bullet holes — the wings, the fuselage, the tail. The instinct was to reinforce those areas. The statistician Abraham Wald saw it differently: the holes showed where a plane could be hit and still come home. The places with no holes — the engines, the cockpit — were where the lost planes had been struck. Reinforce the gaps, he argued, not the marks.
This is survivorship bias: we draw conclusions from the things that made it through the filter and never see the ones that did not. We study successful founders and copy their habits, ignoring the identical habits of thousands who failed. We admire old buildings and call past craftsmanship superior, forgetting the flimsy ones that already collapsed.
The pattern is a hole in the data, not in the analysis. The missing observations are invisible by definition — which is exactly why they are so easy to forget, and so dangerous to ignore.