Survivorship bias

David McRaney wrote an excellent article on Survivorship bias. The crux of the bias is that people tend to make the mistake of only analyzing survivors of a situation or problem, not paying attention to those that did not survive because they’re no longer visible:

Survivorship bias pulls you toward bestselling diet gurus, celebrity CEOs, and superstar athletes… Colleges and conferences prefer speakers who shine as examples of making it through adversity, of struggling against the odds and winning. The problem here is that you rarely take away from these inspirational figures advice on what not to do, on what you should avoid, and that’s because they don’t know. Information like that is lost along with the people who don’t make it out of bad situations or who don’t make it on the cover of business magazines – people who don’t get invited to speak at graduations and commencements and inaugurations.

In short, the advice business is a monopoly run by survivors. As the psychologist Daniel Kahneman writes in his book Thinking Fast and Slow, “A stupid decision that works out well becomes a brilliant decision in hindsight.”

My favorite part was his demonstration of the bias with a fantastic story from World War II:

As with the torpedo problem, the top brass explained what they knew, and the Panel presented the problem to Wald and his group. How, the Army Air Force asked, could they improve the odds of a bomber making it home? Military engineers explained to the statistician that they already knew the allied bombers needed more armor, but the ground crews couldn’t just cover the planes like tanks, not if they wanted them to take off. The operational commanders asked for help figuring out the best places to add what little protection they could. It was here that Wald prevented the military from falling prey to survivorship bias, an error in perception that could have turned the tide of the war if left unnoticed and uncorrected. See if you can spot it.

The military looked at the bombers that had returned from enemy territory. They recorded where those planes had taken the most damage. Over and over again, they saw the bullet holes tended to accumulate along the wings, around the tail gunner, and down the center of the body. Wings. Body. Tail gunner. Considering this information, where would you put the extra armor? Naturally, the commanders wanted to put the thicker protection where they could clearly see the most damage, where the holes clustered. But Wald said no, that would be precisely the wrong decision. Putting the armor there wouldn’t improve their chances at all.

I won’t spoil the surprise – even knowing what the bias is, I didn’t guess why it was a bad decision.