Following up on the previous post showing the tag coverage of the NLTK 2.0b9 default tagger on the treebank corpus, below are the same metrics applied to the conll2000 corpus, using the
analyze_tagger_coverage.py script from nltk-trainer.
NLTK Default Tagger Performance on CoNLL2000
The default tagger is 93.9% accurate on the conll2000 corpus, which is to be expected since both treebank and conll2000 are based on the Wall Street Journal. You can see all the metrics shown below for yourself by running
python analyze_tagger_coverage.py conll2000 --metrics. In many cases, the Precision and Recall metrics are significantly lower than 1, even when the Found and Actual counts are similar. This happens when words are given the wrong tag (creating false positives and false negatives) while the overall tag frequency remains about the same. The
CC tag is a great example of this: the Found count is only 3 higher than the Actual count, yet Precision is 68.75% and Recall is 73.33%. This tells us that the number of words that were mis-tagged as
CC, and the number of
CC words that were not given the
CC tag, are approximately equal, creating similar counts despite the false positives and false negatives.
Unknown Words in CoNLL2000
The conll2000 corpus has 0 words tagged with
-NONE-, yet the default tagger is unable to identify 50 unique words. Here’s a sample: boiler-room, so-so, Coca-Cola, top-10, AC&R, F-16, I-880, R2-D2, mid-1992. For the most part, the unknown words are symbolic names, acronyms, or two separate words combined with a “-“. You might think this can solved with better tokenization, but for words like F-16 and I-880, tokenizing on the “-” would be incorrect.
Missing Symbols and Rare Tags
The default tagger apparently does not recognize parentheses or the
SYM tag, and has trouble with many of the more rare tags, such as
UH. These failures highlight the need for training a part-of-speech tagger (or any NLP object) on a corpus that is as similar as possible to the corpus you are analyzing. At the very least, your training corpus and testing corpus should share the same set of part-of-speech tags, and in similar proportion. Otherwise, mistakes will be made, such as not recognizing common symbols, or finding
-RRB- tags where they do not exist.