Survivorship bias

[[File:Survivorship-bias.svg|thumb|right|225px|Chart of observed damage patterns on [[World War II]] bombers. But only bombers that returned to base were observed. Note the areas that don't show damage.]]
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{{cquote|Once you are familiar with the idea of survivorship bias, you can start spotting it everywhere. For example, a gym might feature those who’ve toned up quickly as a result of going to their facilities but, of course, what they never show is those who signed up but achieved no more than a depleted bank account. Another example is when a company is tempted to copy the strategy of a rival. A policy of "radical transparency" that includes blunt public feedback may work for Netflix and firing the 10% most "under-performing" staff might have been effective at General Electric. But before adopting any of these policies where you work, you should be wary of only looking at their use in successful companies.|||Brendan Miller, "How ‘survivorship bias’ can cause you to make mistakes", ''[[BBC News]]''.<ref>[https://www.bbc.com/worklife/article/20200827-how-survivorship-bias-can-cause-you-to-make-mistakes How ‘survivorship bias’ can cause you to make mistakes]. Miller, Brendan. ''BBC News.'' 28 August 2020.</ref>}}

'''Survivorship bias''' is a [[cognitive bias]] that occurs when focusing on entities that made it past a selection process, while overlooking those that didn't. This can lead to incorrect conclusions due to missing information. It's a specific type of [[selection bias]]. An example of survivorship bias in action would be to draw overly optimistic conclusions from analyzing the performance of businesses in a certain sector using data that excludes businesses which failed or never successfully started up. 

A famous story about Diagoras of Melos, a 5th century BCE Greek [[atheist]] [[philosopher]], shows him criticizing the efficacy of prayer by noting that people who prayed at sea and survived disasters were around to tell of it, while people who prayed and ''died'' during disasters were obviously no longer around to say anything.<ref>{{cite book|first=Jennifer Michael|last=Hecht|author=Jennifer Michael Hecht|title=Doubt: A History|publisher=Harper San Francisco|year=2003|chapter=Whatever Happened to Zeus and Hera?, 600 BCE–1 CE|pages=9–10|isbn=0-06-009795-7|chapter-url=https://books.google.com/books?id=QJb16_AAePkC&pg=PA10}}</ref>

==An in-depth example==
The following comes courtesy of ''You Are Not So Smart''.<ref>Specifically [http://youarenotsosmart.com/2013/05/23/survivorship-bias/ this article].</ref>

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,{{efn|Probably a bit less brilliant when you realize that the surviving planes tended to not be hit in the engines or cockpit…}} a [[Hungary|Hungarian]]-born [[Jew]]ish [[Statistics|statistician]] who fled [[Europe]] to work for the [[US]] military during [[World War II]], which ''also'' goes to show that [[Adolf Hitler|one shouldn't try to]] [[Pol Pot|kill your best thinkers]].{{efn|Or anybody else, for that matter. But then, this lesson is ''hopefully'' more obvious than the first one.}}

==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 [[Goddidit|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 [[art]]istic 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.{{efn|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.}} Used in this way, survivorship bias can lead to [[nostalgia]] for an [[Good old days|imagined glorious past]], hence the pop-culture terms 'nostalgia filter' or 'nostalgia goggles' which describe this effect.<ref>[http://tvtropes.org/pmwiki/pmwiki.php/Main/NostalgiaFilter Nostalgia Filter], ''TV Tropes''.</ref>

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]]<ref>http://www.sciencedirect.com/science/article/pii/S1047279713000896</ref> or silica<ref>http://oem.bmj.com/content/64/10/681</ref> 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 Inc.|Apple Computer]], which became a billion-dollar business, but that the failures made by those imitating Jobs' path to success were ignored.<ref>[http://www.scientificamerican.com/article/how-the-survivor-bias-distorts-reality/ Scientific American — How the Survivor Bias Distorts Reality]</ref> Also, Apple owes much of its success to Steve Wozniak, who did go to college for electrical engineering.

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|pt=Viés de sobrevivência}}

==Notes==
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==References==
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[[Category:Cognitive biases]]

Categories: Cognitive biases