# Survivorship bias

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Diagoras, who is called the atheist, being at Samothrace, one of his friends showed him several pictures of people who had endured very dangerous storms; "See," says he, "you who deny a providence, how many have been saved by their prayers to the Gods." "Ay," says Diagoras, "I see those who were saved, but where are those painted who were shipwrecked?"
—Cicero, On the Nature of the Gods

Survivorship bias is a cognitive bias that occurs when focusing on what made it past a threshold, while overlooking what didn't. It's a specific type of selection bias.

## An in-depth example

The following comes courtesy of You Are Not So Smart.[1]

Suppose you're trying to help the military decide how best to arm their planes for future bombing runs. They let you look over the planes that made it back, and you note that some areas get shot heavily, while other areas hardly get shot at all. So, you should increase the armor on the areas that get shot, right?

Wrong! These are the planes that got shot and survived. It stands to reason that on some planes, the areas that were chewed to bits were not essential, whereas the areas where you don't see any damage did get shot on other planes, but those planes didn't survive. So those are the areas you reinforce. This was the brilliant deduction of Abraham Wald,[note 1] a Hungarian-born Jewish statistician who fled Europe to work for the US military during World War II, which also goes to show that one shouldn't try to kill your best thinkers.[note 2]

## Other examples

One might look at sturdy old buildings and compare to modern constructions, which are often cheaply done and routinely leak, and assume that people back in the day knew how to build things to last. But you are ignoring all the terrible buildings that were either torn down or subject to acts of God; only the best built buildings would survive for centuries. Rome may have buildings that survived for millennia (with constant repairs), but virtually none of the original housing remains as anything more than a construction hazard.

Survivorship bias is also at play when considering the quality of artistic works throughout history. It's easy to look at Shakespeare and think that writers today are much less intelligent than they were in his day, but there were also plenty of writers of Shakespeare's day whose work wasn't as good, and so either didn't survive into the modern era or lacked the influence on Anglophone discourse that Shakespeare achieved.[note 3] Used in this way, survivorship bias can lead to nostalgia for an imagined glorious past, hence the pop-culture terms 'nostalgia filter' or 'nostalgia goggles' which describe this effect.[2]

Fans of foreign films may argue that their production tops those of Hollywood films, or that they are simply more exemplary of the "art of cinema". However, one must consider that films well-received in their home country are more likely to be ported for an international audience. Similarly, younger fans of classic rock music may only be exposed to the music that people who lived through the era liked; there are many hits that you won't find on today's classic rock radio, YouTube compilation, or your parent's record collection.

Survivorship bias can obscure the effects of workplace exposure upon health problems. When new employees with prior exposure to, say, asbestos[3] or silica[4] are inappropriately combined with employees without prior exposure, the apparent exposure effect is reduced. Employees exposed at prior jobs become ill sooner than previously unexposed employees, making exposure of the current employees appear negligible.

Michael Shermer of Scientific American cites survivor bias in books about business success and rags-to-riches stories. In one example, he observes that Steve Jobs dropped out of university to start up Apple Computer, which became a billion-dollar business, but that the failures made by those imitating Jobs' path to success were ignored.[5] Also, Apple owes much of its success to Steve Wozniak, who did go to college for electrical engineering.

 Se você procura pelo artigo em Português, ver Viés de sobrevivência.

## Notes

1. Probably a bit less brilliant when you realize that the surviving planes tended to not be hit in the engines or cockpit…
2. Or anybody else, for that matter. But then, this lesson is hopefully more obvious than the first one.
3. But note that this is not the whole story: Shakespeare and Chaucer both wrote before there was a concept of intellectual property and large parts were unoriginal. Comparing Shakespeare with more original works written later is a bad comparison, considering he enhanced ideas that were already tried-and-true.