Neural Machine Translation has achieved state-of-the-art performance for several language pairs using a combination of parallel and synthetic data. Synthetic data is often generated by back-translating sentences randomly sampled from monolingual data using a reverse translation model. While back-translation has been shown to be very effective in many cases, it is not entirely clear why. In this work, we explore different aspects of back-translation, and show that words with high prediction loss during training benefit most from the addition of synthetic data. We introduce several variations of sampling strategies targeting difficult-to-predict words using prediction losses and frequencies of words. In addition, we also target the contexts of difficult words and sample sentences that are similar in context. Experimental results for the WMT news translation task show that our method improves translation quality by up to 1.7 and 1.2 Bleu points over back-translation using random sampling for German-English and English-German, respectively.

该研究探讨了逆向翻译的不同方面，并表明在训练期间预测损失高的单词最能从合成数据的添加中获益。使用以困难预测单词的预测损失和单词频率的采样策略，以及类似背景的选取句子的策略比随机采样的逆向翻译方法在WMT新闻翻译任务中提高了翻译质量。在德语-英语和英语-德语方向上，翻译质量分别提高了1.7和1.2 BLEU分数。

定向困难词汇的神经机器翻译反向抽样