A Simple Baseline to Semi-Supervised Domain Adaptation for Machine Translation
Table 2: Translation accuracy (BLEU) under different settings. The second and third columns are source and target domains respectively. “DE”, “EN”, and “RO” denote German, English, and Romanian, respectively. DALI and DAFE results are the best results from the original papers, except that numbers marked by † are from our re-implementation. Settings (7) IBT+Src and (8) IBT+Back uses the out-of-domain data and back-translated data to minimize the supervised machine translation loss, respectively. (9) “MT (Sup.)” results are obtained by training an NMT model on the supervised target domain data.
['[EMPTY]', '[BOLD] Methods', '[BOLD] DE to EN  [BOLD] LAW', '[BOLD] DE to EN  [BOLD] LAW', '[BOLD] DE to EN  [BOLD] MED', '[BOLD] DE to EN  [BOLD] MED', '[BOLD] DE to EN  [BOLD] IT', '[BOLD] DE to EN  [BOLD] IT', '[BOLD] DE to EN  [BOLD] WMT-14', '[BOLD] DE to EN  [BOLD] WMT-14', '[BOLD] DE to EN  [BOLD] WMT-14', '[BOLD] RO to EN  [BOLD] WMT-16', '[BOLD] RO to EN  [BOLD] WMT-16', '[BOLD] RO to EN  [BOLD] WMT-16']
Adapting between Specific Domains In this table, the second row lists the source domains whereas the third row shows the target domains. From this table, we see that the unadapted baseline model, Unadapted, performs very poorly, verifying the previous statement that current NMT models cannot generalize well to test data from a new domain. In contrast, the copy method, Copy, and back-translation method, Back, can significantly improve the adaptation performance, with Back showing much superior performance. To be noted, Back even outperforms the other two baselines: DALI and DAFE, by a large margin in the majority of cases, although it was proposed earlier and is much simpler. Notably, IBT only needs the target domain non-parallel data but can still substantially outperform those baselines that rely on the parallel source domain data (e.g., Back, DALI, DAFE), indicating that previous methods have not exhausted the potential contained within the target domain data.