Accurate Unlexicalized Parsing

What it is about

The paper asks how far a PCFG with no lexical items can go on the Penn Treebank. A vanilla treebank grammar parses badly because its independence assumptions are too strong. Adding simple linguistic state splits, such as parent annotation, marking of base noun phrases, tag splits for function words and horizontal markovization, breaks those assumptions, and each split is kept only if it improves F1 on held-out data. The final unlexicalized grammar scores 86.3 F1 on section 23 for sentences of up to 40 words, which is above the early lexicalized PCFGs and close to the state of the art, and it is smaller, faster to parse and easier to replicate. The paper shows that lexical head statistics are not the only source of accuracy.

Ideas

• I1. en/src/ptag.rs: split the tags of the ambiguous words (prepositions and subordinators, past participle against past tense, particles against adverbs) by the tag of the head or the next word, as the paper's tag splits do; measure UPOS and XPOS on EWT test (UPOS 94.74 now) and on cv of the train folds.
• I2. en/src/parse.rs: add the tag of the head's own head (the grandparent in the partial tree) as an arc feature, the analogue of parent annotation; measure cv LAS on the train folds.
• I3. en parse-eval with word identities removed (tags and shapes only): report the LAS drop, which measures how far the en parser depends on lexical items, the quantity the paper's argument turns on.

For Mova

The splits in I1 target the words where the tagger is most often unsure, so the chapters are 10-word-classes and 12-pronouns-determiners-numerals, with 07-punctuation-capitals-numbers for the punctuation tags. The code is en/src/ptag.rs with the TnT prior in en/src/tag.rs, and en/src/parse.rs for idea I2. Measure first: the lexical ablation of idea I3, as it tells whether tag splits or features are the cheaper gain.

Cite

The paper itself, not this card. BibTeX from the library's record of the work. DOI 10.3115/1075096.1075150

@inproceedings{klein2003accurate,
  title = {{Accurate Unlexicalized Parsing}},
  author = {Klein, Dan and Manning, Christopher D.},
  booktitle = {Proceedings of the 41st Annual Meeting of the Association for Computational Linguistics},
  year = {2003},
  doi = {10.3115/1075096.1075150},
  url = {https://doi.org/10.3115/1075096.1075150}
}

References

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