On The Evaluation of Machine Translation Systems Trained With Back-Translation
Table 1: BLEU for four language directions measured on source original sentences (X → Y∗) as well as target original sentences (X∗ → Y) for a model trained on parallel data only (OP) as well as a back-translation model (BT). BT performs much better than OP on the reverse portion of the test set but BLEU shows no difference on the direct portion.
['src', 'ref', 'sys', 'en-de', 'de-en', 'en-ru', 'ru-en']
However, on the reverse portion BT does improve, and it does so by very large margins of between 5.7-10.1 BLEU.