What is Survivor Bias?
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Vsauce had a great example on this about WWII planes. Scientists studied plane that came back and noticed they were hit in the wings and tail. They put more armor there but the same number of planes returned because they had survivorship bias. They were only looking at planes that returned, so the e wings and tail were the places in the ship that could get hit but wouldn’t crash the plane. So they put armor everywhere but the wings and tail and as a result more planes came home. Pretty cool part of history I think
Ever hear people say “they don’t make X like they used to” and point out their X from 1980 that still works great? And then you look at all the X’s from 1980 and they ALL still work great. So obviously they were really well made, right?
No… it’s just that all the X’s from 1980 that aren’t really well made and still running great are scrapped in landfills so they aren’t around to count. But when you only count the ones that **survived**, it looks like they’re all super reliable.
More generally, it’s when a portion of the population no longer exists and you don’t account for that in your analysis.
When you look at something, you usually assume that it’s a typical one of whatever kind of thing it is. There’s way more typical apples than unusual ones, so whatever apple you’re seeing is probably not unusual. Most airplanes are typical airplanes, most people are normal people, etc.
That assumption can be wrong when it’s unusual for this type of thing to be visible to you. If most deer that come near your house get hit by cars, then a deer showing up in your yard is probably unusually good at avoiding cars. If most bullet holes in planes cause the plane to crash, then the bullet holes on the planes that make it home will be unusually far from the vital parts of the plane.
Survivorship bias is the mistake you make when you assume that the things you’re seeing are normal, but they had to be abnormal to be visible to you. If you think normal people can pull themselves out of poverty by their bootstraps because you’ve heard lectures by rich people who did that, or if you think helmets are making soldiers less safe because a lot more came home with head injuries after helmets were added to the uniform, that’s survivorship bias. You’re failing to account for all the people who stayed poor and never got invited to give lectures, and all the soldiers who died of head injuries without helmets but would’ve survived to make it home with them.
Now that you’ve had it explained, you might appreciate [this comic](https://xkcd.com/1827/), which is a joke about survivorship bias.
I once had a boss that said: “we used to ride in the back of the station wagon with no seatbelts and we were fine!” I’m not sure if she was joking or not but you never hear from those who weren’t fine to confirm or deny the safety of wearing a seatbelt.
You have excellent examples here, but I’ll add one:
You know the Boomer meme about how kids are soft now? Back in *their day*, they didn’t have all this safety equipment. Seat belts, no lawn darts, BB gun wars, minimal football helmets, fireworks, Mom had a cigarette and a vodka tonic with every meal while pregnant, they used to play with loaded guns on the train tracks blindfolded, yadda yadda, and they turned out fine. You know that one?
Everyone who says that, did. That risky behavior never bit them, so they’ve come to the conclusion that it actually wasn’t all that risky. *They’re fine*.
You rarely hear from the ones who didn’t turn out fine, but that doesn’t mean things always worked out.
Example: Men make better Partners in Law firms. I know this because all the law partners at top firms are men.
In reality, most top law school graduates are women. Most of the prestigious entry level positions go to women. However, women leave the profession in massive numbers, they go work in non-profit, they switch careers entirely, and some decide to become mothers.
Men survive in the legal profession, survivor bias leads us to believe they are at the top because they are the best.
The aircraft armor example has been mentioned; I would like to add the “infantry helmet” story.
Back in the day (early WW I?) Infantry soldiers usually wore cloth caps in combat. As explosive artillery shells came more into use, lots of soldiers got head injuries, many of them fatal. The Wise Ones said “Let’s have our soldiers wear protective steel helmets to reduce head injuries in combat.”
After this, medical records indicated that a great many more head injuries were being treated. “Oh, No” said the Wise Ones. “Use of helmets increases the number of head injuries in combat.”
“No,” said the Very Wise One. “More soldiers are surviving head injuries and getting treatment because of helmets.”
The adventure stories you hear are all about “the chosen one” because all the kids who went off on an epic adventure only to get immediately stabbed by highwaymen never had stories written about them.
When we were kids we didn’t wear bike helmets and we’re fine.
Of course the kids that aren’t fine aren’t here to disagree.
You can do anything if you just believe in yourself. Look at me, everyone told me I would fail.
I worked for years at low pay jobs, making money for my acting classes, everyone told me to get an education, but I believed in myself, and that is why I made it.
Substitute hard work in an example to become millionaire or astronaut.
This is the story we hear all the time, but we don’t see 10s of thousands or millions who did not make it.
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