Unsupervised Part-of-Speech Tagging in the Large

  • Authors:
  • Chris Biemann

  • Affiliations:
  • Microsoft Corporation / Powerset, San Francisco, USA 94110

  • Venue:
  • Research on Language and Computation
  • Year:
  • 2009

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Abstract

Syntactic preprocessing is a step that is widely used in NLP applications. Traditionally, rule-based or statistical Part-of-Speech (POS) taggers are employed that either need considerable rule development times or a sufficient amount of manually labeled data. To alleviate this acquisition bottleneck and to enable preprocessing for minority languages and specialized domains, a method is presented that constructs a statistical syntactic tagger model from a large amount of unlabeled text data. The method presented here is called unsupervised POS-tagging, as its application results in corpus annotation in a comparable way to what POS-taggers provide. Nevertheless, its application results in slightly different categories as opposed to what is assumed by a linguistically motivated POS-tagger. These differences hamper evaluation procedures that compare the output of the unsupervised POS-tagger to a tagging with a supervised tagger. To measure the extent to which unsupervised POS-tagging can contribute in application-based settings, the system is evaluated in supervised POS-tagging, word sense disambiguation, named entity recognition and chunking. Unsupervised POS-tagging has been explored since the beginning of the 1990s. Unlike in previous approaches, the kind and number of different tags is here generated by the method itself. Another difference to other methods is that not all words above a certain frequency rank get assigned a tag, but the method is allowed to exclude words from the clustering, if their distribution does not match closely enough with other words. The lexicon size is considerably larger than in previous approaches, resulting in a lower out-of-vocabulary (OOV) rate and in a more consistent tagging. The system presented here is available for download as open-source software along with tagger models for several languages, so the contributions of this work can be easily incorporated into other applications.