Discriminative Reranking of Discourse Parses Using Tree Kernels
Summary
The paper tackles RST-style discourse parsing. Existing parsers, including the then state-of-the-art system of Joty et al. (2013), mostly rely on local decisions and miss long-range structural dependencies between discourse tree (DT) constituents. The authors modify that probabilistic parser so it outputs k-best DT hypotheses. They then rerank these hypotheses with Support Vector Machines in a preference-ranking setup. The key ingredient is a family of discourse tree kernels (DiscTK), built by combining standard tree kernels (STK, PTK) with new tree encodings of DTs (JRN and SRN). This lets the reranker implicitly use every subtree fragment as a feature, with no manual feature design. On RST-DT, sentence-level parsing f-score rises from 79.77% to 82.15%. Plugging this reranker into the document-level parser raises its accuracy from 55.8% to 57.3%, a new state of the art. The study shows that structural features matter for discourse parsing and that tree kernels offer a practical way to capture them.
Key points
- Data and setup: experiments use the RST Discourse Treebank (RST-DT) with manual EDU segmentation, on the standard test split plus 5-fold cross-validation.
- Sentence-level gains: the best reranker (k=4–5) reaches 82.15% f-score, a relative error reduction of about 11.8%. The 5-best oracle is 88.09%, and human agreement is around 83%.
- Document-level gains: accuracy improves from 55.8% to 57.3% (p < 0.01). However, k-best oracles at this level are low (56.91% for 5-best, 57.65% for 20-best), which limits how much reranking can help.
- Representations and kernels: SRN outperforms JRN, and STK works better than PTK. Feature-vector rerankers using basic features or bag-of-relations bring no real gain, whereas tree fragments do.
- Error analysis: the reranker struggles with small DTs of two or three EDUs whose relations are semantically close (e.g., Summary vs. Elaboration, Cause vs. Elaboration). The authors conclude that richer semantic representations are needed.
Cited by
- Fast Rhetorical Structure Theory Discourse Parsing
- Two Practical Rhetorical Structure Theory Parsers
- Joint Syntacto-Discourse Parsing and the Syntacto-Discourse Treebank
Cite
@inproceedings{joty2014discriminative,
title = {{Discriminative Reranking of Discourse Parses Using Tree Kernels}},
author = {Joty, Shafiq and Moschitti, Alessandro},
booktitle = {Proceedings of the 2014 Conference on Empirical Methods in Natural Language Processing (EMNLP)},
year = {2014},
doi = {10.3115/v1/d14-1219},
url = {https://doi.org/10.3115/v1/d14-1219}
}References
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