Syntax for Semantic Role Labeling, To Be, Or Not To Be
Summary
Some recent neural semantic role labeling (SRL) systems perform very well without any syntactic input. This raises the question of whether dependency parses still help. He et al. examine this for dependency-based SRL.
They split the task into two steps, predicate disambiguation and argument labeling. Each step uses a BiLSTM encoder topped by a deep MLP with highway connections. Before labeling, an extended k-order pruning algorithm selects argument candidates. It climbs from the predicate to the root of the dependency tree and, at each node, gathers descendants up to distance k. A small k keeps only syntactically close words. A very large k effectively passes the whole sentence, so one framework covers both syntax-aware and syntax-agnostic modes.
Each word is represented by:
- a predicate indicator embedding;
- CNN+BiLSTM character features;
- word, lemma, POS and dependency-label embeddings.
The model sets new best scores on CoNLL-2008/2009 for English and Chinese. The authors also use the Sem-F1/LAS ratio to compare SRL systems fairly when their parsers differ in quality. They conclude that syntax still matters, although its advantage has shrunk to roughly 1–2 points.
Key points
- Evaluated on CoNLL-2009 English and Chinese with the standard splits. Pre-trained embeddings were GloVe for English and Word2Vec trained on Chinese Wikipedia.
- Architecture details: a 4-layer BiLSTM with hidden size 512 and a 10-layer highway MLP with ReLU. The predicate disambiguation model is identical except that it omits the dependency-relation feature.
- The Sem-F1/LAS ratio separates SRL quality from parser quality. By this ratio, most post-CoNLL-2009 gains came from better parsers rather than better SRL components.
- Noisy syntax (LAS 85–100%) was simulated with a random tree generator. Under this noise, tenth-order pruning stayed stable, while first-order pruning lost about 1–5 F1 because it misses true arguments.
- The authors estimate that roughly 90% LAS input would push Sem-F1 above 90%. They also note that the syntax-aware vs. syntax-agnostic gap has fallen from about 10% to 1–2%.
Institutions
Cite
@inproceedings{he2018syntax,
title = {{Syntax for Semantic Role Labeling, To Be, Or Not To Be}},
author = {He, Shexia and Li, Zuchao and Zhao, Hai and Bai, Hongxiao},
booktitle = {Proceedings of the 56th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)},
year = {2018},
doi = {10.18653/v1/p18-1192},
url = {https://doi.org/10.18653/v1/p18-1192}
}References
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