MP-Boost: a multiple-pivot boosting algorithm and its application to text categorization

AdaBoost.MH is a popular supervised learning algorithm for building multi-label (aka n-of-m) text classifiers. AdaBoost.MH belongs to the family of 'boosting' algorithms, and works by iteratively building a committee of 'decision stump' classifiers, where each such classifier is trained to especially concentrate on the document-class pairs that previously generated classifiers have found harder to correctly classify. Each decision stump hinges on a specific 'pivot term', checking its presence or absence in the test document in order to take its classification decision. In this paper we propose an improved version of AdaBoost. MH, called MP-Boost, obtained by selecting, at each iteration of the boosting process, not one but several pivot terms, one for each category. The rationale behind this choice is that this provides highly individualized treatment for each category, since each iteration thus generates, for each category, the best possible decision stump.We present the results of experiments showing that MP-Boost is much more effective than AdaBoost.MH. In particular, the improvement in effectiveness is spectacular when few boosting iterations are performed, and (only) high for many such iterations. The improvement is especially significant in the case of macroaveraged effectiveness, which shows that MP-Boost is especially good at working with hard, infrequent categories.