As can be seen, the base rate P(H) is ignored in this equation, leading to the base rate fallacy. A base rate is a phenomenon's basic rate of incidence. The base rate fallacy describes how people do not take the base rate of an event into account when solving probability problems. This was explicitly tested by Dawes, Mirels, Gold and Donahue (1993) who had people judge both the base rate of people who had a particular personality trait and the probability that a person who had a given personality trait had another one. For example, participants were asked how many people out of 100 answered true to the question "I am a conscientious person" and also, given that a person answered true to this question, how many would answer true to a different personality question. They found that participants equated inverse probabilities (e.g., P(conscientious|neurotic)=P(neurotic|conscientious)) even when it was obvious that they were not the same (the two questions were answered immediately after each other).
