Learning a two-stage SVM/CRF sequence classifier

  • Authors:
  • Guilherme Hoefel;Charles Elkan

  • Affiliations:
  • University of California, San Diego, La Jolla, CA, USA;University of California, San Diego, La Jolla, CA, USA

  • Venue:
  • Proceedings of the 17th ACM conference on Information and knowledge management
  • Year:
  • 2008

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Abstract

Learning a sequence classifier means learning to predict a sequence of output tags based on a set of input data items. For example, recognizing that a handwritten word is "cat", based on three images of handwritten letters and on general knowledge of English letter combinations, is a sequence classification task. This paper describes a new two-stage approach to learning a sequence classifier that is (i) highly accurate, (ii) scalable, and (iii) easy to use in data mining applications. The two-stage approach combines support vector machines (SVMs) and conditional random fields (CRFs). It is (i) highly accurate because it benefits from the maximum-margin nature of SVMs and also from the ability of CRFs to model correlations between neighboring output tags. It is (ii) scalable because the input to each SVM is a small training set, and the input to the CRF has a small number of features, namely the SVM outputs. It is (iii) easy to use because it combines existing published software in a straightforward way. In detailed experiments on the task of recognizing handwritten words, we show that the two-stage approach is more accurate, or faster and more scalable, or both, than leading other methods for learning sequence classifiers, including max-margin Markov networks (M3Ns) and standard CRFs.