Minimum Imputed-Risk: Unsupervised Discriminative Training for Machine Translation

Zhifei Li, Ziyuan Wang, Jason Eisner, Sanjeev Khudanpur, Brian Roark

Abstract: Discriminative training for machine translation has been well studied in the recent past. A limitation of the work to date is that it relies on the availability of high-quality in-domain bilingual text for supervised training. We present an unsupervised discriminative training framework to incorporate the usually plentiful target-language monolingual data by using a rough "reverse" translation system. Intuitively, our method strives to ensure that probabilistic "round-trip" translation from a target-language sentence to the source-language and back will have low expected loss. Theoretically, this may be justified as (discriminatively) minimizing an imputed empirical risk. Empirically, we demonstrate that augmenting supervised training with unsupervised data improves translation performance over the supervised case for both IWSLT and NIST tasks.