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Machine Translation Evaluation for Arabic using Morphologically-enriched Embeddings

Paper · 2016 · Francisco Guzmán, Houda Bouamor, Ramy Baly et al.

Summary

Automatic metrics for translation into morphologically rich languages handle three cases poorly: they cannot reliably separate wrong word choices, acceptable rephrasings, and inflection errors such as broken agreement or wrong case. The authors address this for Arabic. They adapt a feed-forward pairwise neural network, originally built for English MT evaluation, that decides which of two hypotheses better matches a reference. The inputs are sentence embeddings built at several levels of analysis. Lexical levels are surface tokens, normalized tokens and lemmas, segmented with MADAMIRA under the ATB scheme. Morpho-syntactic levels are sequences of POS tags from four tagsets of very different granularity. Cosine similarities and n-gram metrics enter the network as extra features. Combining lexical and rich morpho-syntactic embeddings raised correlation with human pairwise judgments by roughly 75% over the previous state of the art. The finest tagset, Buckwalter POS, was the strongest single representation. This matters because metrics are used to tune systems as well as score them. A metric that notices agreement errors can steer translation into rich-morphology languages more effectively.

Key points

  • Architecture: The input is a triple (reference, hypothesis 1, hypothesis 2). Three tanh hidden groups model each pairwise interaction, and a sigmoid output says which hypothesis is better.

    • Skip arcs feed cosine similarities and AL-BLEU, METEOR and NIST scores straight to the output.
    • Training uses SGD with adagrad and L2 regularization, implemented in Theano.
  • Scoring a single translation: Each translation is compared against an "average" translation built from the training data, running the network in both directions because it is not symmetric. The result is rescaled to [0, 1].

  • Tagsets: CATiBPOS has 6 base tags, Stanford POS 32 (via Stanford CoreNLP), Kulick POS 43 and Buckwalter POS 485. The first three come from MADAMIRA.

  • Single-representation results (Kendall's τ on TEST):

    RepresentationKendall's τ
    Buckwalter POS25.49
    TOKEN24.35
    NORM23.22
    LEMMA21.17
    Kulick16.25
    Stanford10.90
    CATiB5.41

    The best combination was Buckwalter POS + TOKEN.

  • Expressiveness check: The spread (standard deviation) of cosine similarity scores across a representation correlates with its τ at ρ = 0.94. Lexical representations lose some expressiveness because of OOV tokens (about 1.3%).

Interesting ideas

  • I1. Instead of producing an absolute score, train a judge to pick the better of two candidates against a reference, then obtain an absolute score by comparing each candidate with an averaged "typical" candidate in both directions.
  • I2. A sentence made only of very fine morphological tags, with no words at all, carries enough signal to detect agreement errors in gender and number. It was the best single feature, which shows that rich tags are a meaningful view of a sentence on their own.
  • I3. Using one coarse tag for every negative particle let the model penalize a hypothesis that wrongly introduced negation more heavily than one that merely chose a different word. Abstracting closed-class words into function categories exposes meaning shifts that surface tokens hide.
  • I4. Before training anything, a representation's usefulness can be estimated from how widely its similarity scores vary across a test set. Representations that yield nearly constant similarities carry little information.
  • I5. Lexical and morpho-syntactic similarities capture complementary aspects, roughly meaning versus grammatical form, and combining them beat both n-gram metrics and either view alone.

For Mova

How these ideas could serve Mova, our small open language model — its files, stages and tests:

  • I1 (learn): Witness-parser disagreements could be resolved by a pairwise judge that ranks two candidate trees or answers, with the margin against an average candidate serving as a calibrated confidence for silver data.
  • I2 (tokens): The morphology output of crate en could be exported as a tag-sequence fingerprint and used to check number and person agreement between subject and verb before parsing.
  • I3 (frames): Polarity and modality markers (not, never, will, might) could be mapped to shared categories so frame comparison immediately flags a flipped negation instead of treating it as a lexical difference.
  • I4 (learn): This is a cheap screening test for candidate features or embedding levels, run alongside negative controls to discard uninformative views early.
  • I5 (back): The text → graph → text compressor could be evaluated by comparing regenerated text with the original at both the lemma level and the morphological-tag level, catching paraphrase drift and inflection errors separately.

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

 Fields, topics and concepts

Cite

Cite as Francisco Guzmán, Houda Bouamor, Ramy Baly and Nizar Habash. 2016. Machine Translation Evaluation for Arabic using Morphologically-enriched Embeddings. Proceedings of COLING 2016, the 26th International Conference on Computational Linguistics: Technical Papers.

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

@inproceedings{guzman2016machine,
  title = {{Machine Translation Evaluation for Arabic using Morphologically-enriched Embeddings}},
  author = {Francisco Guzmán, Houda Bouamor, Ramy Baly and Nizar Habash},
  booktitle = {Proceedings of COLING 2016, the 26th International Conference on Computational Linguistics: Technical Papers},
  year = {2016},
  url = {https://aclanthology.org/C16-1132/}
}

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