WHAT IS A "SURVIVORSHIP BIAS"?

I want you to look at the image here

This is the image of a plane with bullet holes in World war -2

The army wanted armor to protect their planes but the question was, “Where should they put it?”

When they plotted out the damage these planes were incurring, it was spread out, but largely concentrated around the tail, body and wings.

Now here's the question that you need to answer - Where would you put the armor?

Obviously the most-hit areas of the plane needed additional armor, right?

Abraham Wald, a statistician at the Statistical Research Group made a glaring observation - the military would make a terrible mistake by upgrading the armor along these sections of the plane. Why? Because the military was only looking at the damage on returned planes

They hadn’t saw the damage on planes that didn’t return.

Planes that didn’t return were actually the ones that sustained damage in ways not seen on returned planes i.e their engines.

Unlike the body, tail, and wings, the engine was extremely vulnerable. Once hit there, planes went down, and they didn’t make it back home to have their damage never accounted for

This logical error of concentrating on things that made it past some selection process and overlooking those that did not, typically because of their lack of visibility is called a *survival bias*

This was inspired from @haltakov's tweet where he explained about cognitive bias https://twitter.com/haltakov/status/1311694231602421761?s=19