Fast logistic regression for text categorization with variable-length n-grams

  • Authors:
  • Georgiana Ifrim;Gökhan Bakir;Gerhard Weikum

  • Affiliations:
  • Max-Planck Institute for Informatics, Saarbrücken, Germany;Google Switzerland GmbH, Zürich, Switzerland;Max-Planck Institute for Informatics, Saarbrücken, Germany

  • Venue:
  • Proceedings of the 14th ACM SIGKDD international conference on Knowledge discovery and data mining
  • Year:
  • 2008

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Abstract

A common representation used in text categorization is the bag of words model (aka. unigram model). Learning with this particular representation involves typically some preprocessing, e.g. stopwords-removal, stemming. This results in one explicit tokenization of the corpus. In this work, we introduce a logistic regression approach where learning involves automatic tokenization. This allows us to weaken the a-priori required knowledge about the corpus and results in a tokenization with variable-length (word or character) n-grams as basic tokens. We accomplish this by solving logistic regression using gradient ascent in the space of all ngrams. We show that this can be done very efficiently using a branch and bound approach which chooses the maximum gradient ascent direction projected onto a single dimension (i.e., candidate feature). Although the space is very large, our method allows us to investigate variable-length n-gram learning. We demonstrate the efficiency of our approach compared to state-of-the-art classifiers used for text categorization such as cyclic coordinate descent logistic regression and support vector machines.