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That old fan at home has been spinning for thirty years. Grown-ups like to say: things were just built better back then. But the fans from back then that broke were thrown out long ago. You only see the one that didn't. This little trick of the mind has a name. Let's try a small experiment first, then I'll tell you.
Imagine 1,024 people in a coin-flip contest. Before each flip, everyone guesses heads or tails. Guess wrong and you're out. Each round, about half go home. After one round, about 500 are left; after two, about 250. After ten rounds, roughly one person remains.
He guessed right ten times in a row. A reporter asks: what's your secret? He says: a glass of milk every morning, and a deep breath before each guess. Sounds convincing. But plenty of the 1,023 who lost drink milk too.
Nobody interviews them. The champion just got lucky. In World War Two, the US military ran into the same kind of puzzle. The most famous version goes like this: the bombers that came back had lots of holes in the wings and body. Near the engines, hardly any.
Many thought: add armor wherever the holes are. But the statistician Abraham Wald asked: what about the planes that never came back? Planes hit in the engines mostly didn't make it home. So the spots with few holes are exactly the ones that matter most.
The real history is messier than the story. But the idea is still used today. Looking only at what's left, and forgetting what got filtered out. That mistake is called survivorship bias. One last example, still without an answer.
In 1987, an animal hospital in New York studied cats that fell from buildings. Cats that fell from above the sixth floor were hurt less than those from lower floors. Some say a cat falling longer relaxes and spreads out. Others say the worst-hurt cats never reached the hospital at all. Which is it? Scientists are still arguing.
Seen survivorship bias in your own life? Tell me in the comments.
You only see
the one that didn’t break.
Grown-ups like to say “things were built better back then.” But the fans from back then that broke were thrown out long ago — nobody keeps a broken fan in the living room. The ones still around were the toughest ones to begin with.
Before each flip, everyone guesses heads or tails. Guess wrong and you’re out. About half leave each round: 512, 256, 128… After ten rounds, roughly one person is left.
Someone will always
guess right ten times.
The champion just got lucky. With over a thousand people guessing, one of them going all the way is no surprise. Look only at the winner, and luck starts to look like skill.
The most famous version goes like this: bombers that made it home had lots of holes in the wings and body, but hardly any near the engines. Many thought: add armor wherever the holes are.
Statistician Abraham Wald asked: what about the ones that didn’t come back?
Planes hit in the engines mostly didn’t make it home, so you rarely see engine holes on the ones that did. The spots with few holes are exactly the ones that matter most.
The real history is messier: in 1943, at the Statistical Research Group at Columbia University, Wald wrote a set of reports on how to estimate where a plane is most vulnerable from the damage on planes that returned. Details like “the military then armored the engines” have no reliable source.
Seeing what’s left,
forgetting what got cut.
That mistake is called survivorship bias: everything you get to see has already passed through a filter — and you forget the filter is there.
When you see one, ask: what about the ones I can’t see?
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