Simple Fusion: Return of the Language Model

Felix Stahlberg, James Cross, Veselin Stoyanov


Abstract
Neural Machine Translation (NMT) typically leverages monolingual data in training through backtranslation. We investigate an alternative simple method to use monolingual data for NMT training: We combine the scores of a pre-trained and fixed language model (LM) with the scores of a translation model (TM) while the TM is trained from scratch. To achieve that, we train the translation model to predict the residual probability of the training data added to the prediction of the LM. This enables the TM to focus its capacity on modeling the source sentence since it can rely on the LM for fluency. We show that our method outperforms previous approaches to integrate LMs into NMT while the architecture is simpler as it does not require gating networks to balance TM and LM. We observe gains of between +0.24 and +2.36 BLEU on all four test sets (English-Turkish, Turkish-English, Estonian-English, Xhosa-English) on top of ensembles without LM. We compare our method with alternative ways to utilize monolingual data such as backtranslation, shallow fusion, and cold fusion.
Anthology ID:
W18-6321
Volume:
Proceedings of the Third Conference on Machine Translation: Research Papers
Month:
October
Year:
2018
Address:
Brussels, Belgium
Editors:
Ondřej Bojar, Rajen Chatterjee, Christian Federmann, Mark Fishel, Yvette Graham, Barry Haddow, Matthias Huck, Antonio Jimeno Yepes, Philipp Koehn, Christof Monz, Matteo Negri, Aurélie Névéol, Mariana Neves, Matt Post, Lucia Specia, Marco Turchi, Karin Verspoor
Venue:
WMT
SIG:
SIGMT
Publisher:
Association for Computational Linguistics
Note:
Pages:
204–211
Language:
URL:
https://aclanthology.org/W18-6321
DOI:
10.18653/v1/W18-6321
Bibkey:
Cite (ACL):
Felix Stahlberg, James Cross, and Veselin Stoyanov. 2018. Simple Fusion: Return of the Language Model. In Proceedings of the Third Conference on Machine Translation: Research Papers, pages 204–211, Brussels, Belgium. Association for Computational Linguistics.
Cite (Informal):
Simple Fusion: Return of the Language Model (Stahlberg et al., WMT 2018)
Copy Citation:
PDF:
https://aclanthology.org/W18-6321.pdf