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The magnitude, efficiency, and scope of learning ill-defined categories without feedback training were investigated for a wide range of stimulus-distortion conditions. In Experiment 1, subjects classified either low-, medium-, high-, or mixed-level distortions under feedback or no-feedback training and then received a common transfer test containing old, new, prototype, and unrelated stimuli. In Experiment 2, the training patterns were multidimensionally scaled, and a measure of categorical structure was derived for each training condition. The results indicated that feedback training was consistently superior to no-feedback conditions and that category size was important only for the feedback conditions. Minimal learning occurred in the no-feedback conditions, except when the training set was highly structured. A simple relationship was proposed, relating ease of learning to degree of categorical structure. The benefits of feedback were discussed in terms of choice reduction and schema formation, with the latter viewed as instrumental in preventing the formation of idiosyncratic groupings based on adventitious feature similarities in patterns from other categories.The present study was concerned with the criticality of feedback on the acquisition of illdefined categories. Specifically, do conditions exist that preclude learning unless feedback is provided? Furthermore, if such conditions exist, can they be identified in terms of the structural properties of the categories? In the present study, performance was contrasted for a variety of situations, and the utility of a recent measure of conceptual structure (Homa, Rhoads, & Chambliss, 1979) was assessed as a predictor of category learning in the presence and in the absence of feedback.There is no dispute that some learning may occur in the absence of feedback. Numerous experiments involving the free sorting of patterns (

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