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Target Language Adaptation of Discriminative Transfer Parsers

Paper · 2013 · Oscar Täckström, Ryan McDonald and Joakim Nivre

Summary

The paper addresses dependency parsing for languages that have no treebank of their own. It does this by transferring parsers trained on other languages. Earlier state-of-the-art work by Naseem et al. (2012) shared parameters selectively across source languages based on typology. That work relied on a weak generative model whose supervised accuracy stays well below discriminative parsers.

The authors bring selective sharing into a delexicalized, arc-factored discriminative graph-based parser. They split its features into a language-generic part and parts tied to WALS word-order traits and language family. They then adapt this transfer parser to the target language using unlabeled target text. Their learning objective accepts ambiguous labelings, meaning a set of allowed heads per word rather than a single tree. This supports two variants:

  • Self-training: arcs filtered by the base parser's marginals.
  • Ensemble-training: those arcs plus the predictions of another transfer parser.

Both variants also relexicalize the model. Evaluated on 16 CoNLL treebanks, the final model beats Naseem et al. on 15 of them. The work shows that rich discriminative models and cheap unlabeled data can push cross-lingual transfer further.

Key points

  • Setup: 16 target languages, following the data and setup of Naseem et al. (2012).

    • The remaining treebanks serve as source training data.
    • Coarse "universal" POS tags use the Naseem et al. (2010) mapping.
    • Stripped CoNLL training sets act as unlabeled target data.
  • Typological features: WALS features 81A, 85A, 86A and 87A drive selective sharing. Features 88A and 89A were dropped because they barely vary across the studied languages.

  • Gap motivating the work (supervised, averaged over 16 languages):

    • Naseem et al.'s generative model: 67.1%.
    • Lexicalized arc-factored discriminative parser: 84.1%.
    • Delexicalized discriminative parser: 78.9%.
  • Average transfer accuracy with unlabeled target data:

    ModelAvg. accuracy
    NBG+EM60.4
    Family (base parser)62.0
    Viterbi self-training62.4
    AAST64.0
    AAET65.4

    The Family model alone beats Naseem et al. on 12 of 16 languages. AAET cuts errors by 13% relative to NBG+EM.

  • Ambiguity level and headroom:

    • With the threshold σ = 0.95 (and λ = 1, since tuning was impossible), dependents keep about 1.4–3.2 candidate heads on average.
    • The oracle accuracy of AAST's ambiguous labelings is 75.7%.

Interesting ideas

  • I1. Typology-conditioned feature templates. Order-sensitive features can be conjoined with a language's WALS values, while order-free attachment features stay shared. This lets a single model pool many source languages without pretending the target is a blend of them. The approach could suit transfer to Ukrainian from a mixed set of Slavic and non-Slavic sources.
  • I2. Ambiguous labelings instead of single 1-best trees. The base parser's high-marginal arcs are kept as a set of acceptable heads, and the model is trained to put mass on any of them. Unlike plain Viterbi self-training, this gave consistent gains rather than occasional drops. It is a general recipe for noisy pseudo-labels in any structured task.
  • I3. Ensembles as label sets. Predictions from a different parser can simply be added to the allowed-arc set. This merges heterogeneous systems without needing their probabilities, which is useful when only another system's outputs are available.
  • I4. Relexicalize during adaptation. Training the self-trained model with lexical features on target text is where the gain comes from. Delexicalized self-training moved the average only from 62.0 to 62.1, so unlabeled data pays off mainly when it unlocks new feature types.
  • I5. Report the supervised ceiling and the oracle of pseudo-labels. Comparing each transfer model with its supervised upper bound, and measuring how often the ambiguous set contains the gold arc, shows how much room is left. This diagnostic is cheap and portable to other low-resource evaluations.

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

 Institutions

Cite

Cite as Oscar Täckström, Ryan McDonald and Joakim Nivre. 2013. Target Language Adaptation of Discriminative Transfer Parsers. Proceedings of the 2013 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies.

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

@inproceedings{tackstrom2013target,
  title = {{Target Language Adaptation of Discriminative Transfer Parsers}},
  author = {Oscar Täckström, Ryan McDonald and Joakim Nivre},
  booktitle = {Proceedings of the 2013 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies},
  year = {2013},
  url = {https://aclanthology.org/N13-1126/}
}

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