Feature-Rich Translation by Quasi-Synchronous Lattice Parsing
What it is about
The paper presents a log-linear translation model that puts phrase features, target n-gram and dependency-syntax features, and tree-to-tree configuration features into one score. The decoder is lattice parsing: a sausage lattice of candidate target words is parsed with a quasi-synchronous dependency grammar of the German source tree. Non-local features such as trigram and phrase features are handled by cube summing, an approximate method. On the BTEC German-English task with about 82K training sentences, adding phrase features to the base model raises BLEU from about 0.37 to 0.45, and adding target syntax reaches about 0.50. Adding the configuration features gives about 0.51. Restricting which configurations are allowed matters: strict synchronous alignment scores 0.40, and allowing parent-child and sibling configurations reaches the best value. Permitting every configuration does not help further. The lexicon keeps the top three translations for each source word, and 191 of 934 development sentences could not be reached by the model.
Ideas
- I1. Coverage as a checked count: the decoder cannot enforce that each source word is used once, so it needs coverage features. The translator's reader should report the same residual: the count of source tokens read zero times and read twice, per verse, as a hard check on the meta-world.
- I2. Keep the lexicon with a reachability count: the paper's top-three rule (ranked by p(s|t) times p(t|s)) leaves some sentences unreachable. For each merge in the translator's lexicon, report how many verses lose their reading when it is dropped, instead of only the merge count.
- I3. Configurations as separate features: count the configuration class of each meta-token pair and give each class its own weight in the encoder score, not a single penalty. This is a count version of the paper's tree-to-tree features and fits open problem 1.
- I4. Pseudolikelihood as a check, not a method: the paper trains with pseudolikelihood on a dev set. A count-based check of the same kind is to compare the encoder's score of the gold verse against the score of the verse with one token changed, over the Bible pairs, and to report the share where the gold wins.
For Mova
The code is tools/translate/src/profile.rs (the encoder score) and tools/translate/src/meta.rs (the reader). The decoder itself, with lattice parsing and cube summing, is not for Mova, since the translator has no search at test time that needs it. The feature grouping and the coverage check carry over as counts over tables that are ordered by key.
Cite
@inproceedings{gimpel2009feature,
title = {{Feature-Rich Translation by Quasi-Synchronous Lattice Parsing}},
author = {Gimpel, Kevin and Smith, Noah A.},
booktitle = {Proceedings of the 2009 Conference on Empirical Methods in Natural Language Processing},
year = {2009},
doi = {10.3115/1699510.1699539},
url = {https://doi.org/10.3115/1699510.1699539}
}References
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