Training and Scaling Preference Functions for Disambiguation
Paper · 1994 · Hiyan Alshawi and David Carter
Summary
Broad-coverage rule-based analysers generate many competing readings per sentence. To rank them, they combine several preference functions in a weighted sum. Those weights were usually set by experts through trial and error, and the tuning had to be redone for each new domain. Alshawi and Carter learn these "scaling factors" automatically: a least-squares fit supplies starting values, and hill climbing then refines them. The experiments use the Core Language Engine (CLE), which outputs quasi logical form (QLF) analyses, applied to the ATIS corpus. The roughly 650 ATIS trees in the Penn Treebank were too few for reliable results, so the authors semi-automatically built skeletal bracketings for over 4,000 sentences. Each analysis was then scored by how well its constituents overlapped with these trees. The learned weights significantly outperform a laborious hand-tuned set. A second study compares semantic collocation functions over word senses. A newly defined one beats variants based on mutual information and likelihood ratios, lowering disambiguation error from about 30% to about 10%. The study shows that preference weighting can be trained cheaply and that collocation measures should be evaluated together with the other functions.
Key points
- Method: the weights of a linear combination of preference functions are initialized by squared-error minimization and then improved by hill climbing. This replaces expert hand tuning, which the learned factors significantly outperform.
- Data: the ATIS-2 training corpus supplied 4615 sentences belonging to classes A and D, none longer than 15 words. The CLE yielded an acceptable constituent structure for 4092 of them (about 89%). Skeletal trees with only two categories, A (argument) and P (predication), were created for these sentences through an interactive tool.
- Training score: an analysis earns credit for segments it shares with the treebank tree and is penalized for extra segments. The chosen weights were a1=1, a2=10 and a3=0.
- System setup: about 30 preference functions were tested, and 20 were kept for the spoken language translator (1 collocation-based, 19 others). These include attachment counts and SynRules, a log-probability estimate over syntactic rules.
- Collocations: the best word-sense collocation function beats both the mutual-information and likelihood-ratio variants and the syntactic-rule-probability function, reducing error from about 30% to about 10%. The method was later reused for reordering N-best speech hypotheses (Rayner et al. 1994).
Institutions
Fields, topics and concepts
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Cite
Cite as Hiyan Alshawi and David Carter. 1994. Training and Scaling Preference Functions for Disambiguation. Computational Linguistics.
@article{alshawi1994training,
title = {{Training and Scaling Preference Functions for Disambiguation}},
author = {Hiyan Alshawi and David Carter},
journal = {Computational Linguistics},
year = {1994},
url = {https://aclanthology.org/J94-4005/}
}References
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