Unleashing the Power of Neural Discourse Parsers - A Context and Structure Aware Approach Using Large Scale Pretraining
Summary
RST-style discourse parsing builds constituency trees over EDUs and helps tasks such as summarization and sentiment analysis. Neural parsers have had trouble beating feature-engineered systems, mainly because gold treebanks are small: RST-DT has only 385 documents. Guz, Huber and Carenini propose a compact bottom-up shift-reduce parser that predicts tree structure and nuclearity but not relation labels. At every step, a classifier chooses the next action from two inputs: a RoBERTa encoding of the relevant stack and queue spans, and numerical organizational features that describe document layout. Training happens in two stages. The parser is first pretrained on MEGA-DT, an automatically derived "silver-standard" treebank built from Yelp'13 sentiment data. It is then fine-tuned on human-annotated RST-DT or Instr-DT. The model sets a new SOTA on most measures for both corpora, and MEGA-DT pretraining improves results further. The work shows that pretrained language models can replace manual feature engineering and that large noisy treebanks are useful supervision for robust cross-domain discourse parsing.
Key points
- MEGA-DT (Huber and Carenini, 2020) contains over 250,000 Yelp'13 documents. This is roughly three orders of magnitude more than RST-DT or Instructional-DT. Because it lacks relation labels, the parser handles only structure and nuclearity (NN, NS, SN).
- Model: the parser uses a lightweight RoBERTa (82M parameters) plus explicit structural features inside a linear-time shift-reduce framework.
- Results: the parser is best on all reported measures except Instr-DT structure, where it roughly matches Joty et al. (2015). It clearly outperforms Wang et al. (2017) on RST-DT structure, and with pretraining it substantially improves Instr-DT nuclearity.
- Ablation:
- Removing the organizational features lowers scores noticeably.
- Removing RoBERTa drops performance far below the baselines.
- A randomly initialized RoBERTa keeps structure accuracy competitive but performs poorly on nuclearity.
- Future work:
- larger language models;
- relation prediction;
- top-down parsing;
- pretraining on the full MEGA-DT, which the authors could not yet do due to compute limits;
- silver-standard data for PDTB and other sentiment corpora such as IMDB and Amazon.
Cited by
- A Simple and Strong Baseline for End-to-End Neural RST-style Discourse Parsing
- eRST: A Signaled Graph Theory of Discourse Relations and Organization
Cite
@inproceedings{guz2020unleashing,
title = {{Unleashing the Power of Neural Discourse Parsers - A Context and Structure Aware Approach Using Large Scale Pretraining}},
author = {Guz, Grigorii and Huber, Patrick and Carenini, Giuseppe},
booktitle = {Proceedings of the 28th International Conference on Computational Linguistics},
year = {2020},
doi = {10.18653/v1/2020.coling-main.337},
eprint = {2011.03203},
archivePrefix = {arXiv},
url = {https://doi.org/10.18653/v1/2020.coling-main.337}
}References
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