Probability of positive instances (.125, .25, or .5), the number of relevant stimulus dimensions (1, 2, 3, or 4), and sex of S were factorially combined in a concept-identification task. Trials to criterion were found to increase linearly with the number of relevant stimulus dimensions and log-sub(2) of the probability of positive instances. The number of positive instances to criterion increased linearly with the number of relevant stimulus dimensions, and the slope of the function increased as the probability of positive instances increased. The results indicate that negative instances serve an important role in the process of identifying the concept. The results are discussed in terms of the way in which Ss process information in the task. (PsycINFO Database Record (c) 2006 APA, all rights reserved)