David McRaney explains “survivorship bias” by using an example from WWII:
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 [statistician Abraham] Wald said no, that would be precisely the wrong decision. Putting the armor there wouldn’t improve their chances at all.
Do you understand why it was a foolish idea? The mistake, which Wald saw instantly, was that the holes showed where the planes were strongest. The holes showed where a bomber could be shot and still survive the flight home, Wald explained. After all, here they were, holes and all. It was the planes that weren’t there that needed extra protection, and they had needed it in places that these planes had not. The holes in the surviving planes actually revealed the locations that needed the least additional armor. Look at where the survivors are unharmed, he said, and that’s where these bombers are most vulnerable; that’s where the planes that didn’t make it back were hit.
The commanders’ mistake is very common:
After any process that leaves behind survivors, the non-survivors are often destroyed or rendered mute or removed from your view. If failures becomes invisible, then naturally you will pay more attention to successes. Not only do you fail to recognize that what is missing might have held important information, you fail to recognize that there is missing information at all.
You must remind yourself that when you start to pick apart winners and losers, successes and failures, the living and dead, that by paying attention to one side of that equation you are always neglecting the other. If you are thinking about opening a restaurant because there are so many successful restaurants in your hometown, you are ignoring the fact the only successful restaurants survive to become examples. Maybe on average 90 percent of restaurants in your city fail in the first year. You can’t see all those failures because when they fail they also disappear from view.