Base rates: how common is it to begin with?

A clue can be useful and still lead us toward the wrong conclusion if we forget how common the possibilities are. An alert may detect most faulty items while many of its alerts concern items that are actually fine.

A base rate is the starting frequency of a condition in the relevant group. It gives the context needed to interpret a clue. This lesson works through that relationship with counts, so you can see where each part of the answer comes from.