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Extracting Parallel Sentences from Comparable Corpora using Document Level Alignment

Summary

Statistical MT systems depend heavily on parallel training data, which is scarce for most language pairs and domains. The authors treat Wikipedia as a comparable corpus: articles on the same topic are already paired across languages by user-made "interwiki" links, so the task reduces to finding parallel sentences inside each linked article pair. Instead of the usual binary parallel/non-parallel classifier, they propose a Maximum Entropy ranking model. For each source sentence, it picks a target sentence or "null". They also propose a first-order linear-chain CRF that aligns the whole document at once. The CRF exploits the fact that parallel sentences tend to appear in clusters. Features come from IBM Model 1 and HMM word alignments, sentence length and position, distortion, and Wikipedia markup (links, shared image captions, lists). A lexicon induced from comparable text adds further features. Experiments on Spanish-, German- and Bulgarian-English show that the extracted sentences raise BLEU, most strongly on Wikipedia-domain text. This makes Wikipedia a practical parallel-data source for language pairs with few resources.

Key points

  • Wikipedia already provides document alignment through interwiki links. Table 1 gives the counts of English-paired articles, for example French 496K, German 488K, Spanish 311K and Czech 87K.
  • The extracted data were large: roughly 1.69M German-English, 1.91M Spanish-English and 146K Bulgarian-English sentence pairs were added to training.
  • Adding the extracted data to medium-sized systems improved BLEU in every language pair. On the new Wikitest set, gains were +6.1 (es-en), +5.2 (de-en) and +10.1 (bg-en).
  • On Wikitest, a medium system plus the Wikipedia extracts matched or beat the large system alone. Adding the extracts to the large system still gave +2.2 to +3.5 BLEU on Wikitest, while changes on Test A were small.
  • The authors released manually aligned document pairs and the hand-cleaned "Wikitest" test set, with about 500 sentences per language pair.

Interesting ideas

  • I1. Rank instead of classify. Choosing the best match (or "null") for each source sentence avoids the heavy class imbalance of labelling all sentence pairs as positive or negative. The same framing could help any one-to-at-most-one matching task, such as entity linking or record deduplication.
  • I2. Global, non-monotonic document alignment. A CRF with distortion features rewards alignments that follow the previous aligned sentence. This captures the tendency of parallel content to come in clusters, and the idea could transfer to aligning translated web pages or subtitles.
  • I3. Use structural metadata as weak signals. Matching hyperlinks are weighted by their inverse frequency, and shared image captions or list membership act as indicators. The image and list indicators take a negative value when only one side matches. Such cheap markup cues could be reused when mining bilingual sites or Ukrainian–English Wikipedia pairs.
  • I4. Induce a lexicon from non-parallel article pairs. A word-level model trained on linked articles adds translations that the seed parallel data lacks. This raises recall without the drawbacks of self-training, which can only learn words that already occur in extracted parallel sentences.
  • I5. Build an in-domain, hand-cleaned test set. Testing only on Europarl-like data would have hidden most of the benefit. A small, manually edited Wikipedia test set showed where the extracted data actually helps.

Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.

Fields: Natural language processing

Topics: Machine translation

Cited by

110 papers of our library cite it (of the 140884 whose bibliographies we read); 8 of them have a page here, 102 not yet.

Cite

The paper itself, not this page. BibTeX from the library's record of the work.

@inproceedings{smith2010extracting,
  title = {{Extracting Parallel Sentences from Comparable Corpora using Document Level Alignment}},
  author = {Smith, Jason R. and Quirk, Chris and Toutanova, Kristina},
  booktitle = {Human Language Technologies: The 2010 Annual Conference of the North American Chapter of the Association for Computational Linguistics},
  year = {2010},
  url = {https://aclanthology.org/N10-1063/}
}

References

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