Similarity , Frequency , and Category Representations

This article studies the joint roles of similarity and frequency in determining graded category structure. Perceptual classification learning experiments were conducted in which presentation frequencies of individual exemplars were manipulated. The exemplars had varying degrees of similarity to members of the target and contrast categories. Classification accuracy and typicality ratings increased for exemplars presented with high frequency and for members of the target category that were similar to the high-frequency exemplars. Typicality decreased for members of the contrast category that were similar to the high-frequency exemplars. A frequency-sensitive similarity-to-exemplars model provided a good quantitative account of the classification learning and typicality data. The interactive relations among similarity, frequency, and categorization are considered in the General Discussion. Among the most well-established findings in the categori-zation literature is that categories have "graded structures" Rather than all instances of a category being "equal," it appears that certain instances are better examples than others. For example, people reliably rate a robin as a better example of the category birds than they rate a penguin. Various experimental operations converge on the view that categories have graded structures, including typicality ratings, errors in classification learning, reaction time in speeded classification, and exemplar production. Why are some instances of a category better examples than others? There has been widespread agreement since the work of Rosch and Mervis (1975) that a major determinant of graded category structure involves stimulus similarity. Generally speaking, the more similar an instance is to the other members of its category and the less similar it is to members of contrast categories, the higher will be the typicality rating given to that instance. So, for example, whereas robins are highly similar to numerous other instances of the category birds, penguins are relatively dissimilar to other bird instances. Another variable that may play an important role in determining graded category structure is stimulus frequency. It seems plausible that as the frequency with which a person experiences an instance as an example of a category increases, the "goodness" of that instance as an example of the category will also increase. Thus, robins may be rated as highly typical birds because they are frequently experienced examples of the category birds. iment 2) conducted a category learning condition in which frequency was inversely related to similarity. Stimuli that were highly similar to other members of the category were presented less frequently than were stimuli that were relatively dissimilar …