Anecdote

Back in World War II a man was asked to figure out a way to better protect planes and pilots by adding more armor to Allied aircraft.
So he basically drew out the planes and looked at all of the bullet holes in the planes that returned from missions. Every bullet hole meant a dot on his drawing of each respective plane, and eventually large areas of the pictures became dotted.

Now, you'd think he'd have said, "Put all of the armor on the spots that seem get hit the most."

Instead he said this - put the armor on the spots that we never see bullet holes, because those holes are only found in the planes that never make it home.