Base rate effects on the interpretations of probability and frequency expressions

Two studies were run to determine whether the interpretations of statements or forecasts using vague probability and frequency expression such as likely, improbable, frequently, or rarely, were sensitive to the base rates of the events involved, In the first experiment, professional weather forecasters judged event probabilities in situations drawn from a medical context. In the second experiment, students judged matched forecast scenarios of common semantic content that differed only in prior probability (as determined by an independent group of subjects). Results were (a) the interpretations of forecasts using neutral (e.g., possible) and high probability or frequency terms (e.g. usually) were strong, positive functions of base rate, while the interpretations of forecasts using low terms (e.g. rarely) were much less affected by base rates; (b) in the second experiment interpretations of forecasts appeared to represent some kind of average of the meaning of the expression and the base rate.