Squares receive higher category ratings when the smaller sizes are presented more frequently than the larger sizes. This shift in the rating scale is greater when there are either fewer categories (the Category Effect) or more stimuli. Similar shifts were obtained whether the stimuli were presented successively for judgment or simultaneously. The Category Effect also occurred when subjects were not told how many categories to use until after the contextual stimuli had been presented. A simple range-frequency model describes most of the shifts in scale by variations in a single weighting parameter. However, these shifts are predicted by an elaborated model in which the number of representations of any stimulus in working memory is limited by a principle of consistent assignment of each stimulus to a single category. This elaborated model correctly predicts the disappearance of the Category Effect when contexts are manipulated by varying the spacing of stimulus values rather than by varying their relative frequencies.