The base-rate fallacy: Contrasting processes and outcomes of group and individual judgment

The study compared the use of base-rate and individuating information by groups and individuals. Two hundred and forty subjects were presented with either a high or a low base rate and with descriptions of three individuals that varied in the extent to which they sounded like members of professional categories. Subjects were asked to judge the probabilities that the individuals belonged to a particular category. Subjects made their judgments either as groups, as coacting individuals, as individuals who “thought out loud,” or as individuals who did not orally report their thoughts. Results indicate that for descriptions that sounded like members of categories, the probability judgments of groups were farther from the base rate than those of individuals. Thus, group discussion appears to amplify the tendency to judge primarily by representativeness when the individuating information is informative. Conversely, for descriptions that did not sound like members of professional categories, groups were more affected by the base rate and their probability judgments were closer to it than those of individuals. The data from the group discussions and the individual protocols were consistent with the probability estimates.