Mitigating Metric Bias in Minimum Bayes Risk Decoding

Geza Kovacs, Daniel Deutsch, Markus Freitag


Abstract
While Minimum Bayes Risk (MBR) decoding using metrics such as COMET or MetricX has outperformed traditional decoding methods such as greedy or beam search, it introduces a challenge we refer to as metric bias. As MBR decoding aims to produce translations that score highly according to a specific utility metric, this very process makes it impossible to use the same metric for both decoding and evaluation, as any improvement might simply be due to reward hacking rather than reflecting real quality improvements. In this work we demonstrate that compared to human ratings, neural metrics not only overestimate the quality of MBR decoding when the same metric is used as the utility metric, but they also overestimate the quality of MBR/QE decoding with other neural utility metrics as well. We also show that the metric bias issue can be mitigated by using an ensemble of utility metrics during MBR decoding: human evaluations show that MBR decoding using an ensemble of utility metrics outperforms a single utility metric.
Anthology ID:
2024.wmt-1.109
Volume:
Proceedings of the Ninth Conference on Machine Translation
Month:
November
Year:
2024
Address:
Miami, Florida, USA
Editors:
Barry Haddow, Tom Kocmi, Philipp Koehn, Christof Monz
Venue:
WMT
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
1063–1094
Language:
URL:
https://aclanthology.org/2024.wmt-1.109
DOI:
Bibkey:
Cite (ACL):
Geza Kovacs, Daniel Deutsch, and Markus Freitag. 2024. Mitigating Metric Bias in Minimum Bayes Risk Decoding. In Proceedings of the Ninth Conference on Machine Translation, pages 1063–1094, Miami, Florida, USA. Association for Computational Linguistics.
Cite (Informal):
Mitigating Metric Bias in Minimum Bayes Risk Decoding (Kovacs et al., WMT 2024)
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PDF:
https://aclanthology.org/2024.wmt-1.109.pdf