Evaluating Chinese-Vietnamese Low-resource Machine Translation with Back Translation
Table 4: Results of character-based NMT
['Directions Settings', 'zh\\rightarrowvi BLEU', 'zh\\rightarrowvi METEOR', 'vi\\rightarrowzh BLEU', 'vi\\rightarrowzh METEOR']
But we observe that the best scores of both BLEU and METEOR appear in the middle data size (80k) instead of the largest one, which is quite interesting and beyond our expectation. From 60k to 80k data, the scores increase correspondingly, but decrease since then. The similar conclusion is also proved in previous works(stahlberg2018simple). This result seems to indicate that more synthetic data is not always better, especially for some far-distance languages. When it comes to other translation direction, the positive effects of synthetic Chinese data are obvious and the increasing data sizes continue to improve the translation performance, until reaching the best status under the largest data size, about 1 point higher than baseline.