Most accounts of categorization assume the categorization decision for an item to be independent of the categorization decisions for other items. A number of methods are brought to bear on the question of whether this assumption is justified. These methods involve the application of a formal categorization model that explicitly incorporates the independence assumption to categorization data and the subsequent investigation of the residuals for unexplained structure. The residuals reveal multiple departures from independence, suggesting that the independence assumption in many a categorization account should be relaxed. Following this suggestion the applied formal model is extended to allow for dependent categorization decisions. It is explained how the extended model might address the concern that categorization accounts have erred in using similarity as an explanatory construct. It promises to be a significant step towards a categorization-based model of similarity.