What Causes the Failure of Explicit to Implicit Discourse Relation Recognition?
Summary
Classifiers for implicit discourse relations are often trained on explicit examples with their connectives removed, but they do poorly on genuine implicit data. Earlier work blamed linguistic differences between the two types without testing that claim across a corpus. Liu, Wan and Strube identify one concrete cause: label shift. Once a connective is deleted, the remaining arguments may express a different relation than the original annotation. For example, a Temporal example marked by "then" can read as Cause without it.
The authors confirm the shift through a small manual study and then at corpus scale on PDTB 2.0 and PDTB 3.0. Among the factors they examine, the syntactic role of connectives contributes most to the shift. They propose two remedies:
- a cosine-similarity-based label shift metric, used to filter out noisy training examples;
- joint learning, in which a RoBERTa-based model recovers a connective and then classifies the relation.
Combining both methods outperforms strong baselines on PDTB and on the RST-annotated GUM corpus. This suggests that cheap explicit data can be used more reliably for implicit relation recognition.
Key points
- Manual check: Two trained annotators labelled 100 randomly sampled PDTB 2.0 explicit instances with connectives removed. This gave initial evidence of label shift before the corpus-level analysis.
- Setup: PDTB sections 2–20 were used for training, 0–1 for development and 21–22 for testing. Evaluation covered top- and second-level relations, with RoBERTa base plus a linear layer as the classifier. Code is available at github.com/liuwei1206/Exp2Imp.
- Ablation on PDTB: Dropping the filtering step lowered top-level F1 by 5.86% on PDTB 2.0 and 4.98% on PDTB 3.0. Removing joint learning also hurt performance, and the two strategies proved complementary.
- GUM results (v9, DISRPT 2023): Explicit and implicit examples were separated with a heuristic rule. Training on explicit data (E2I-Entire) reached 32.52 F1, compared with 56.81 when training on implicit data. The full method reached 37.56 F1, about 5 points above the E2I baseline.
- Limitations: Only English PDTB- and RST-style corpora were tested. Other frameworks (e.g., SDRT) and other languages are left for future work.
Cite
@inproceedings{liu2024what,
title = {{What Causes the Failure of Explicit to Implicit Discourse Relation Recognition?}},
author = {Liu, Wei and Wan, Stephen and Strube, Michael},
booktitle = {Proceedings of the 2024 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (Volume 1: Long Papers)},
year = {2024},
doi = {10.18653/v1/2024.naacl-long.150},
eprint = {2404.00999},
archivePrefix = {arXiv},
url = {https://doi.org/10.18653/v1/2024.naacl-long.150}
}References
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