Understanding Survivor Bias: The Revealing Story Of A Bullet-Riddled Plane And A Helmet

The longer you stay on Facebook, the closer the chance of you seeing a “we didn’t have seatbelts/safety features/basic concerns for our survival needs when was a child and we still survived” post gets to 100 percent.

If you haven’t encountered them, think of the person who tells you “my gran smoked every day of her life and she made it to 95” or “I used to pound 35 cans of Budweiser before every commute and not once did I die” from Uncle Billy. These are all examples of “survivor bias“, which goes like this: these activities look less dangerous than they are because you are one of the survivors. There’s a reason you don’t hear from people with similar stories, such as “I used to pound 35 cans of Budweiser before driving and died instantly, day one” from Uncle Billy (God rest his soul), and it’s because Uncle Billy is dead.

Let’s move away from the depressing examples, and take a look at a couple that crop up on the Internet from time to time: The helmet and the plane. Wars are a great place to find survivors, hence why these stories both take place during a war. 

The WWI Helmet

This one, though likely apocryphal, goes like this: In WWI, generals began to become alarmed when hospitals started to see a large number of head injuries to soldiers in hospitals. Given the influx, they thought that the problem might be that the newly introduced helmet – the Brodie helmet, specifically – might be causing head injuries. In fact, the reason why they were seeing so many head injuries is that the helmets were allowing far more soldiers to survive.

The Planes

During World War II, the Americans wanted to reduce the casualty rates of their air squadrons. Many planes came back riddled with bullet holes in three main areas: the fuselage, the outer wings, and the tail. They came up with a solution to reinforce the hell out of the areas that had been filled with enemy fire. Which seems logical enough.

However, the reason why this is such a great example of survivor bias is that the only data they had to go on was from the survivors. Before they could start to reinforce the areas, Abraham Wald, a Hungarian-Jewish statistician took a look at the data and realized the flaw in their reasoning. 

In essence, the bullet holes in the fuselage, outer wings, and tails on the planes that survived showed this: if planes could be shot full of bullets in those three areas and make it back, that meant that being shot there wasn’t very dangerous to the planes. If you assume (and you have to make assumptions as you are not seeing all the evidence, as it’s lying somewhere in a war zone) that bullet holes are distributed pretty evenly over planes when you look at all the planes that go out to fight, that means that the ones that didn’t make it back likely had bullet holes in other areas. His solution was to account for the survivor bias, and suggest that they should reinforce all the areas that didn’t have bullet holes, in order to increase survival. 

An earlier version of this article was published in September 2021.

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