The present invention is a system and method for determining whether the best category for an object under investigation is a mixture of preexisting categories, and how the mixture is constituted. This invention is useful both for suggesting the need for new categories, and for a fixed set of categories, determining whether a document should be assigned to multiple categories. The objects of the categorization system are typically, but need not be, documents. Categorization may be by subject-matter, language or other criteria. The invention causes extra information to be stored in a category index, so that the determination of mixed categories using the methods presented here is performed extremely efficiently.