Text classification in a hierarchical mixture model for small training sets
Documents are commonly categorized into hierarchies of topics, such as the ones maintained by Yahoo! and the Open Directory project, in order to facilitate browsing and other interactive forms of information retrieval. In addition, topic hierarchies can be utilized to overcome the sparseness problem in text categorization with a large number of categories, which is the main focus of this paper. This paper presents a hierarchical mixture model which extends the standard naive Bayes classifier and previous hierarchical approaches. Improved estimates of the term distributions are made by differentiation of words in the hierarchy according to their level of generality/specificity. Experiments on the Newsgroups and the Reuters-21578 dataset indicate improved performance of the proposed classifier in comparison to other state-of-the-art methods on datasets with a small number of positive examples.