Another Reason Why Selection Effects Are Important

In his post over at Marginal Revolution Alex Tabarrok reminds us all why selection effects are important to consider in our work.

During WWII, statistician Abraham Wald was asked to help the British decide where to add armor to their bombers. After analyzing the records, he recommended adding more armor to the places where there was no damage.

The RAF was initially confused. Can you explain (without peeking below)?

Because Wald had data only on the planes that returned to Britain he concluded that the bullet holes that he saw were all in places where a plane could be hit and still survive. Wald then surmised that the planes that were shot down were probably hit in different places than those that returned. Thus, Wald recommended adding armor to the places where the surviving planes were lucky enough not to have been hit.