A classic example of selection bias occurred during World War II. During the war, the British were losing many airplanes over enemy territory and therefore decided to add armor plating to their bombers. The armor was not only heavy, but also expensive, so the British decided to only add armor to the most critical areas of the planes, determined by the location of bullet holes in returning aircraft. The areas most commonly marked by bullet holes were the wings, the nose, and the tail. Before the plan was implemented, Austrian economist Abraham Wald reviewed the data and claimed that the British plan was just the opposite of what was needed, and the armor should be added to the only areas not designated for armor by the British plan: the body and the rudder. The British followed Wald’s recommendation and as a result, many fewer planes were shot down. Explain the selection bias in the original British plan.