On the other hand, data-based methods perform adaptation either by combining in-domain and out-of-domain parallel corpora for supervised adaptation (Luong and Manning, 2015;Freitag and Al-Onaizan, 2016) or by generating pseudo-parallel corpora from indomain monolingual data for unsupervised adaptation (Sennrich et al., 2016a;Currey et al., 2017).
in this paper we tackle the task of data-based, unsupervised adaptation, where representative methods include creation of a pseudoparallel corpus by back-translation of in-domain monolingual target sentences (Sennrich et al., 2016a), or construction of a pseudo-parallel indomain corpus by copying monolingual target sentences to the source side (Currey et al., 2017).