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Part of Speech Tagging with NLTK – Part 3

In part 2, I showed how to produce a part-of-speech tagger using Ngram tagging in combination with Affix and Regex tagging, with accuracy approaching 90%. In part 3, I'll use the BrillTagger to get the accuracy up to and over 90%.

Brill Tagging

The BrillTagger is different than the previous taggers. For one, it's not a SequentialBackoffTagger, though it does use an initial tagger, which in our case will be the raubt_tagger from part 2. The BrillTagger uses the initial tagger to produce initial tags, then corrects those tags based on transformational rules. These rules are learned by training with the FastBrillTaggerTrainer and rules templates. Here's an example, with templates copied from the demo() function in nltk.tag.brill.py. Refer to part 1 for the backoff_tagger function and the train_sents, and part 2 for the word_patterns.

import nltk.tag
from nltk.tag import brill

raubt_tagger = backoff_tagger(train_sents, [nltk.tag.AffixTagger,
    nltk.tag.UnigramTagger, nltk.tag.BigramTagger, nltk.tag.TrigramTagger],
    backoff=nltk.tag.RegexpTagger(word_patterns))

templates = [
    brill.SymmetricProximateTokensTemplate(brill.ProximateTagsRule, (1,1)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateTagsRule, (2,2)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateTagsRule, (1,2)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateTagsRule, (1,3)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateWordsRule, (1,1)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateWordsRule, (2,2)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateWordsRule, (1,2)),
    brill.SymmetricProximateTokensTemplate(brill.ProximateWordsRule, (1,3)),
    brill.ProximateTokensTemplate(brill.ProximateTagsRule, (-1, -1), (1,1)),
    brill.ProximateTokensTemplate(brill.ProximateWordsRule, (-1, -1), (1,1))
]

trainer = brill.FastBrillTaggerTrainer(raubt_tagger, templates)
braubt_tagger = trainer.train(train_sents, max_rules=100, min_score=3)

Brill Tagging Accuracy

So now we have a braubt_tagger. You can tweak the max_rules and min_score params, but be careful, as increasing the values will exponentially increase the training time without significantly increasing accuracy. In fact, I found that increasing the min_score tended to decrease the accuracy by a percent or 2. So here's how the braubt_tagger fares against the other taggers.

Conclusion

There's certainly more you can do for part-of-speech tagging with nltk, but the braubt_tagger should be good enough for many purposes. The most important component of part-of-speech tagging is using the correct training data. If you want your tagger to be accurate, you need to train it on a corpus similar to the text you'll be tagging. The brown, conll2000, and treebank corpora are what they are, and you shouldn't assume that a tagger trained on them will be accurate on a different corpus. For example, a tagger trained on one part of the brown corpus may be 90% accurate on other parts of the brown corpus, but only 50% accurate on the conll2000 corpus. But a tagger trained on the conll2000 corpus will be accurate for the treebank corpus, and vice versa, because conll2000 and treebank are quite similar. So make sure you choose your training data carefully.

If you'd like to try to push the accuracy even higher, see part 4, where I compare the braubt_tagger to classifier based taggers, and nltk.tag.pos_tag.

Related posts

• Part of Speech Tagging with NLTK Part 4 – Brill Tagger vs Classifier Taggers
• Part of Speech Tagging with NLTK – Part 2
• Part of Speech Tagging with NLTK – Part 1
• Linguistic and Natural Language Processing Links
• How to Train a NLTK Chunker