A numerical decision task, meant to be a numerical analogue of signal detection, was employed to investigate the nature of the decision rule used for probabilistic categorization. Two conditions were compared, with 12 paid volunteers participating in six 1-hr sessions in Condition 1 and 12 participating in three 1-hr sessions in Condition 2. In both conditions, one of two distributions of five-digit numbers was sampled on each trial (400 trials per session), and the research participant was to decide which distribution was sampled. Specifically, the participant was told that each stimulus represented the height of a person from an artificial population, and the participant was to decide whether that height belonged to a man or a woman. The stimulus distributions were normal and equal in variance; their means were one standard deviation apart. In Condition 1 the participants were free to adopt whatever decision procedure they preferred, including a probabilistic strategy. In contrast, in Condition 2 the participants were forced to adopt a cutoff rule: They reported a five-digit cutoff number before each trial. On each trial, participants were forced to produce a response consistent with the reported cutoff number-that is, the response had to be man if the stimulus number was larger than the reported cutoff and woman if it was smaller.For both conditions, the numbers of deviations from the best fitting staticcutoff rule were computed for each block of SO trials. Although the deviations were found to be significantly more numerous in Condition 1 than in Condition 2, in Condition 1 the numbers of deviations were much closer to those in Condition 2 than to the number of deviations predicted from the best available probabilistic model (Schoeffler, 1965). Furthermore, Conditions 1 and 2 were very similar with respect to other characteristics, such as the effects of practice and the locations of the best fitting static-cutoff point. Sequential analyses of the reported cutoff numbers in Condition 2 revealed that participants did not always shift their cutoffs in the direction predicted by additive-operator models and that, contrary to the predictions of these models, the probabilities and amounts of cutoff shift were not stationary over sessions. In contrast to the additiveoperator models, a new dynamic-cutoff model describing the behavior of an ideal learner was able to account for some of these features of the data, but additional analyses ruled out this model as well.These results led to the following conclusions: (a) In a numerical decision task a dynamic-cutoff rule is a good first approximation to the decision rule adopted by individuals, (b) The only dynamic-cutoff models previously available in the literature-the additive-operator models-are inadequate, (c) A new dynamic-cutoff model-the ideal-learner model-is also inadequate, (d) Individuals tend to shift their cutoffs in the normatively prescribed direction after errors, but tend to shift their cutoffs randomly after correct responses. 427