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Paper · 2005.01196 · ACL · 2020

On the Limitations of Cross-lingual Encoders as Exposed by Reference-Free Machine Translation Evaluation

Yang Gao, Wei Zhao, Steffen Eger, Maxime Peyrard, Robert West, Goran Glavaš

arXiv · PDF · Open in the Atlas

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AIPHES/ACL20-Reference-Free-MT-Evaluation canonical 2 of 2
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bert_encode Ran AIPHES/ACL20-Reference-Free-MT-Evaluation/score_utils.py
code served (permissive licence) · get_code("78010dc51e6ba072")
collate_idf Ran AIPHES/ACL20-Reference-Free-MT-Evaluation/score_utils.py
code served (permissive licence) · get_code("d9cba0455da2e616")
padding Ran this paper's copy was not recorded; identical code first harvested from yuhui-zh15/nlg_metrics
pointer only · get_code("13ef8eeae8570519")

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Abstract

Evaluation of cross-lingual encoders is usually performed either via zero-shot cross-lingual transfer in supervised downstream tasks or via unsupervised cross-lingual textual similarity. In this paper, we concern ourselves with reference-free machine translation (MT) evaluation where we directly compare source texts to (sometimes low-quality) system translations, which represents a natural adversarial setup for multilingual encoders. Referencefree evaluation holds the promise of web-scale comparison of MT systems. We systematically investigate a range of metrics based on state-of-the-art cross-lingual semantic representations obtained with pretrained M-BERT and LASER. We find that they perform poorly as semantic encoders for reference-free MT evaluation and identify their two key limitations, namely, (a) a semantic mismatch between representations of mutual translations and, more prominently, (b) the inability to punish "translationese", i.e., low-quality literal translations. We propose two partial remedies: (1) post-hoc re-alignment of the vector spaces and (2) coupling of semantic-similarity based metrics with target-side language modeling. In segment-level MT evaluation, our best metric surpasses reference-based BLEU by 5.7 correlation points. We make our MT evaluation code available. 1

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