Improving sequence segmentation learning by predicting trigrams

  • Authors:
  • Antal van den Bosch;Walter Daelemans

  • Affiliations:
  • Tilburg University, Tilburg, The Netherlands;University of Antwerp, Antwerp, Belgium

  • Venue:
  • CONLL '05 Proceedings of the Ninth Conference on Computational Natural Language Learning
  • Year:
  • 2005

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Abstract

Symbolic machine-learning classifiers are known to suffer from near-sightedness when performing sequence segmentation (chunking) tasks in natural language processing: without special architectural additions they are oblivious of the decisions they made earlier when making new ones. We introduce a new pointwise-prediction single-classifier method that predicts trigrams of class labels on the basis of windowed input sequences, and uses a simple voting mechanism to decide on the labels in the final output sequence. We apply the method to maximum-entropy, sparse-winnow, and memory-based classifiers using three different sentence-level chunking tasks, and show that the method is able to boost generalization performance in most experiments, attaining error reductions of up to 51%. We compare and combine the method with two known alternative methods to combat near-sightedness, viz. a feedback-loop method and a stacking method, using the memory-based classifier. The combination with a feedback loop suffers from the label bias problem, while the combination with a stacking method produces the best overall results.