Discriminative Instance Weighting for Domain Adaptation in Statistical Machine Translation
Summary
The paper tackles domain adaptation for phrase-based SMT. A large out-of-domain parallel corpus (OUT) is used to help a smaller in-domain one (IN), which is hard when the two differ a lot. The authors do not decide at the level of whole sentences or corpora. Instead, they assign a weight to every OUT phrase pair. The weight is a logistic function of features that estimate how similar the pair is to IN and how much it looks like general language, including the output of an SVM. The weighted counts are MAP-smoothed and then mixed linearly with the IN phrase table. All mixing, smoothing and feature weights are fitted together by maximizing likelihood on a small IN dev set with L-BFGS. The method was tested in two settings:
- Europarl helping the EMEA medical domain (English↔French).
- Non-news NIST09 data helping Chinese→English news.
In both, it beat a wide set of baselines. This shows that fine-grained, cheaply trained weighting can recover the useful part of distant background data.
Key points
- Two experimental settings: EMEA/EP for fren and enfr, and NIST Chinese→English evaluated on NIST06 and NIST08. Results are reported in BLEU.
- Best instance-weighting model:
- beats the same mixture model without instance weights by 0.6–1.8 BLEU;
- is over 2 BLEU above the best non-adapted baseline.
- Baselines include:
- IN+OUT concatenation;
- log-linear combination of separate models tuned with MERT;
- linear and MAP (Dirichlet-prior) mixtures;
- sentence selection by IN-LM perplexity.
- Log-linear combination trails linear/MAP mixtures by a wide margin on EMEA, yet performs comparably on NIST, whose domains are wider and more similar to each other. Linear and MAP mixtures show no large systematic gap.
- Keeping original OUT frequencies and smoothing the weighted counts with a uniform prior (non-zero γ) clearly outperforms the unsmoothed and "flattened" variants.
Interesting ideas
- I1. Separate "general" from "similar". The paper does not only look for OUT data resembling the target domain. It also scores whether an example is general language, which matters when domains are far apart. This is a useful lens for any transfer setup with distant source data.
- I2. Use the overlap as positive labels. An SVM is trained with phrase pairs found in both IN and OUT as positive examples. This yields a "generality" signal without manual annotation and could be reused to label transferable items in other cross-domain tasks.
- I3. Weight at the finest natural unit. Weighting phrase pairs rather than sentences lets general fragments inside domain-specific sentences still contribute. Similar unit-level weighting could suit n-gram LMs or lexicons.
- I4. Tune mixtures by dev-set likelihood instead of MERT/BLEU. Fitting linear mixture weights to a joint phrase-pair distribution taken from the dev set is cheap and stable. It only needs evaluation over that small support, so it runs much faster than metric-driven tuning while still optimizing many parameters at once.
- I5. Perplexity-ranked selection as a simple baseline. Ranking OUT sentence pairs by IN-LM perplexity of the target side, with the cutoff tuned on dev BLEU, is an easy and well-motivated data-selection baseline for any corpus-filtering experiment.
Cited by
- A Survey of Domain Adaptation for Neural Machine Translation
- Measuring Machine Translation Errors in New Domains
- Crossing the Threshold: Idiomatic Machine Translation through Retrieval Augmentation and Loss Weighting
Cite
@inproceedings{foster2010discriminative,
title = {{Discriminative Instance Weighting for Domain Adaptation in Statistical Machine Translation}},
author = {Foster, George and Goutte, Cyril and Kuhn, Roland},
booktitle = {Proceedings of the 2010 Conference on Empirical Methods in Natural Language Processing},
year = {2010},
url = {https://aclanthology.org/D10-1044/}
}References
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