Sampling-Based Approximations to Minimum Bayes Risk Decoding for Neural Machine Translation

Bryan Eikema, Wilker Aziz


Abstract
In NMT we search for the mode of the model distribution to form predictions. The mode and other high-probability translations found by beam search have been shown to often be inadequate in a number of ways. This prevents improving translation quality through better search, as these idiosyncratic translations end up selected by the decoding algorithm, a problem known as the beam search curse. Recently, an approximation to minimum Bayes risk (MBR) decoding has been proposed as an alternative decision rule that would likely not suffer from the same problems. We analyse this approximation and establish that it has no equivalent to the beam search curse. We then design approximations that decouple the cost of exploration from the cost of robust estimation of expected utility. This allows for much larger hypothesis spaces, which we show to be beneficial. We also show that mode-seeking strategies can aid in constructing compact sets of promising hypotheses and that MBR is effective in identifying good translations in them. We conduct experiments on three language pairs varying in amounts of resources available: English into and from German, Romanian, and Nepali.
Anthology ID:
2022.emnlp-main.754
Volume:
Proceedings of the 2022 Conference on Empirical Methods in Natural Language Processing
Month:
December
Year:
2022
Address:
Abu Dhabi, United Arab Emirates
Editors:
Yoav Goldberg, Zornitsa Kozareva, Yue Zhang
Venue:
EMNLP
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
10978–10993
Language:
URL:
https://aclanthology.org/2022.emnlp-main.754
DOI:
10.18653/v1/2022.emnlp-main.754
Bibkey:
Cite (ACL):
Bryan Eikema and Wilker Aziz. 2022. Sampling-Based Approximations to Minimum Bayes Risk Decoding for Neural Machine Translation. In Proceedings of the 2022 Conference on Empirical Methods in Natural Language Processing, pages 10978–10993, Abu Dhabi, United Arab Emirates. Association for Computational Linguistics.
Cite (Informal):
Sampling-Based Approximations to Minimum Bayes Risk Decoding for Neural Machine Translation (Eikema & Aziz, EMNLP 2022)
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PDF:
https://aclanthology.org/2022.emnlp-main.754.pdf