Ideally, a decision maker or a forecaster will combine the outside view and the inside view—or, similarly, statistics plus personal experience. But it’s much better to start with the statistical view, the outside view, and then modify it in the light of personal experience than it is to go the other way around. If you start with the inside view you have no real frame of reference, no sense of scale—and can easily come up with a probability that is ten times too large, or ten times too small.
