Survivorship bias

Bias is a partiality that prevents objective consideration of an issue or situation.

Photo by Markus Spiske on Unsplash

Survivorship bias or survival bias is the logical error of concentrating on the people or things that made it past some selection process and overlooking those that did not, typically because of their lack of visibility. This can lead to some false conclusions in several different ways. It is a form of selection bias.

WW2

The most famous example of survivorship bias dates back to World War Two. At the time, the American military asked mathematician Abraham Wald to study how best to protect airplanes from being shot down. The military knew armour would help, but couldn’t protect the whole plane or would be too heavy to fly well. Initially, their plan had been to examine the planes returning from combat, see where they were hit the worst –  and then reinforce those areas. 

photo credit Martin Grandjean

However,  it wasn't what Abraham Wald said they should do. Why?

Wald realised they had fallen prey to survivorship bias, because their analysis was missing a valuable part of the picture: the planes that were hit but that hadn’t made it back. As a result, the military were planning to armour precisely the wrong parts of the planes. The bullet holes they were looking at actually indicated the areas a plane could be hit and keep flying – exactly the areas that didn't need reinforcing.

Winners and Losers Examples

So, what survivorship bias does for us is make use focus on what we can see (the winners), and ignore what we can’t see (the losers). This, in turn,  seriously distorts our ability to calculate the odds of something happening.

Product And Book Reviews

You read what people say about the product / book, but don't think about the number of people who purchased it, but didn't leave a review at all. Basically, what happens here is that you don't stop to think what's not written in those reviews.

(just) Drop out of school and become - SOMEONE

Photo by Bakhrom Tursunov on Unsplash

A number of today’s billionaires – Bill Gates and Mark Zuckerberg, for example - achieved their success despite never going to or finishing university, a fact that has attracted considerable media attention. 

However, no one writes an in-depth story on the talented actor who never made it big in Hollywood and eventually gave up. 

The media doesn’t interview the dedicated, hard-working CEO of a newly bankrupt company. 

No one makes an awesome YouTube video on the college athlete who sacrificed everything to make it pro and failed. 

The (sad) reality is that the overwhelming number of (often random) failures never make it to you. Instead, you hear about the stories of the successful - and maybe, decide to drop out of school, as well. I mean, even Einstein flunked math! Or maybe he did just fine, but it isn't an appealing story for anyone, is it!

Public Health

To date, the COVID-19 pandemic has claimed well over 5.67 million lives (February, 1st, 2022), and still, a number of people challenge the virus in various ways - from non-existent and common, to artificial and not serious at all. On the other hand, people may think that it is ventilators that kill COVID patients, because a very large number of people who are put on ventilators - sadly, die. Conversely, it implies that if you don't put a patient on a ventilator, their chances of survival are significantly higher. What people do not want to think about is the reason why medical staff decide to put someone on a ventilator in the first place.

courtesy of the decision lab

The danger in basing your understanding of the world on those who have ‘beaten the odds’ or have succeeded taking ‘big risks’ becomes clear if you carefully consider the logic of those phrases. Such people must be unrepresentative of the bigger picture, and thus one should be cautious of emulating them. After all, if everyone was succeeding by taking a big risk, it can’t have been that big a risk, nor the odds that daunting.