Neural Generative Rhetorical Structure Parsing
Summary
Rhetorical Structure Theory (RST) parsing builds a tree over a document's elementary discourse units (EDUs), linking them with rhetorical relations and nuclearity. The main training resource, the English RST-DT, is small, and earlier parsers were all locally normalized discriminative models. These tend to suffer from label bias and to learn less efficiently from limited data. The authors propose the first generative RST parser, a document-level RNNG with bottom-up transitions. It has two actions: one generates an EDU, and one merges the top two stack units under a relation and nuclearity label. EDU generation is scored with a neural language model. Structural decisions use a stack representation that combines neural embeddings with hand-crafted structural features.
The authors show that the standard word-level beam search of Fried et al. (2017) pushes this kind of parser toward left-branching trees. They introduce a bag-level beam search that avoids this bias. With it, the generative model beats an otherwise identical discriminative baseline. It also matches state-of-the-art performance without extra training data, which suggests generative modelling suits low-resource discourse parsing.
Key points
- Data and setup: experiments use RST-DT (347 training documents) with gold EDU segmentation, 18 coarse-grained relations and the nuclearity labels SN, NS and NN. Structural features come from sentence and paragraph boundaries and from spaCy dependency trees.
- Search algorithm: on the development set, bag-level beam search improves on word-level beam search by 6.8 points of unlabelled F1 and 2.9 points of labelled F1. It wins at every beam size tested.
- Generative vs. discriminative: on the test set, the generative model reaches a full-tree F1 of 45.0, compared with 42.4 for the discriminative baseline. That is a gain of 2.6, and it improves every metric by 1.9–2.7 points.
- Comparison with prior parsers: test scores are S 67.1, N 57.4 and R 45.5. The N score is the best of all parsers in Morey et al. (2017)'s replication study. The full-tree score beats every published parser there that uses no additional training data.
- Design findings: bottom-up traversal worked better than top-down, and conditioning only on the stack helped. Neural embeddings alone performed poorly unless structural features were added.
Cited by
- Where Are We in Discourse Relation Recognition?
- Bilingual Rhetorical Structure Parsing with Large Parallel Annotations
- Unleashing the Power of Neural Discourse Parsers - A Context and Structure Aware Approach Using Large Scale Pretraining
- MEGA RST Discourse Treebanks with Structure and Nuclearity from Scalable Distant Sentiment Supervision
- RST Discourse Parsing with Second-Stage EDU-Level Pre-training
- Adversarial Learning for Discourse Rhetorical Structure Parsing
Cite
@inproceedings{mabona2019neural,
title = {{Neural Generative Rhetorical Structure Parsing}},
author = {Mabona, Amandla and Rimell, Laura and Clark, Stephen and Vlachos, Andreas},
booktitle = {Proceedings of the 2019 Conference on Empirical Methods in Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP)},
year = {2019},
doi = {10.18653/v1/d19-1233},
eprint = {1909.11049},
archivePrefix = {arXiv},
url = {https://doi.org/10.18653/v1/d19-1233}
}References
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