Rethinking Round-trip Translation for Automatic Machine Translation Evaluation

A parallel corpus is generally required to automatically evaluate the translation quality using the metrics, such as BLEU, METEOR and BERTScore. While the reference-based evaluation paradigm is widely used in many machine translation tasks, it is difficult to be applied to translation with low-resource languages, as those languages suffer from a deficiency of corpora. Round-trip translation provides an encouraging way to alleviate the urgent requirement of the parallel corpus, although it was unfortunately not observed to correlate with forwarding translation in the era of statistical machine translation. In this paper, we firstly observe that forward translation quality consistently correlates to corresponding round-trip translation quality in the scope of neural machine translation. Then, we carefully analyse and unveil the reason for the contradictory results on statistical machine translation systems. Secondly, we propose a simple yet effective regression method to predict the performance of forward translation scores based on round-trip translation scores for various language pairs, including those between very low-resource languages. We conduct extensive experiments to show the effectiveness and robustness of the predictive models on 1,000+ language pairs. Finally, we test our method on challenging settings, such as predicting scores: i) for unseen language pairs in training and ii) on real-world WMT shared tasks but in new domains. The extensive experiments demonstrate the robustness and utility of our approach. We believe our work will inspire works on very low-resource multilingual machine translation.