Disambiguating Explicit Discourse Connectives without Oracles
Summary
Before any PDTB-style discourse parser can work, it has to decide whether a candidate connective such as "and" or "because" is actually signalling a discourse relation. Errors at this stage carry over to every later step. The widely cited 96.29% accuracy of Pitler and Nenkova (2009) relies on gold-standard parse trees and is averaged over tokens, so a handful of frequent connectives dominate it. The authors re-evaluate the task on the Penn Discourse Treebank (PDTB) and WSJ under more realistic conditions: features come from automatically predicted parses and tags, and scores are macro-averaged across connective types as well as micro-averaged across instances. They compare the P&N feature set, its extension by Lin et al. (2010), and a new lightweight POS+LEX model that uses only words and predicted part-of-speech tags. Under realistic evaluation, performance drops sharply for the parse-based models. POS+LEX beats P&N and is statistically on par with Lin. Cheap, robust features are therefore sufficient, and the rare connectives that matter for applications such as text simplification still need more work.
Key points
- On predicted features, macro-averaged F1 is 80.7 for P&N, 83.6 for Lin and 83.5 for POS+LEX, against a majority baseline of 53.9. A Wilcoxon signed-rank test found the difference between POS+LEX and Lin not significant at p < 0.01.
- Under oracle conditions, P&N reaches 93.0 micro-F1, which falls to 85.3 when macro-averaged and to roughly 80.7 when predicted parses replace gold ones as well.
- Connective frequencies in the PDTB (about 100 explicit types) follow a power law: the top 10 cover over 75% of occurrences and the top 20 over 90%. Still, 70% of WSJ articles with explicit connectives contain at least one outside the top 10, rising to 87% when very short texts are excluded.
- Setup:
- Parses come from the Stanford Parser, trained in a leave-one-section-out fashion, with an average F1 of 85.87%.
- The classifier is maximum entropy, trained on WSJ sections 2–22 with ten-fold cross-validation.
- Without an oracle, the average per-feature conditional entropy of P&N rises by 8.8%, while POS features barely change. For 58 connective types, Lin's parse features introduce at least as many new errors as they fix relative to POS+LEX.
Cited by
- Learning Connective-based Word Representations for Implicit Discourse Relation Identification
- The DISRPT 2023 Shared Task on Elementary Discourse Unit Segmentation, Connective Detection, and Relation Classification
- Comparing PTB and UD information for PDTB discourse connective identification
Cite
@inproceedings{johannsen2013disambiguating,
title = {{Disambiguating Explicit Discourse Connectives without Oracles}},
author = {Johannsen, Anders and Søgaard, Anders},
booktitle = {Proceedings of the Sixth International Joint Conference on Natural Language Processing},
year = {2013},
url = {https://aclanthology.org/I13-1134/}
}References
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