Bidirectional Phrase-based Statistical Machine Translation

This paper investigates the effect of direction in phrase-based statistial machine translation decoding. We compare a typical phrase-based machine translation decoder using a left-to-right decoding strategy to a right-to-left decoder. We also investigate the effectiveness of a bidirectional decoding strategy that integrates both mono-directional approaches, with the aim of reducing the effects due to language specificity. Our experimental evaluation was extensive, based on 272 different language pairs, and gave the surprising result that for most of the language pairs, it was better decode from right-to-left than from left-to-right. As expected the relative performance of left-to-right and rightto-left strategies proved to be highly language dependent. The bidirectional approach outperformed the both the left-toright strategy and the right-to-left strategy, showing consistent improvements that appeared to be unrelated to the specific languages used for translation. Bidirectional decoding gave rise to an improvement in performance over a left-to-right decoding strategy in terms of the BLEU score in 99% of our experiments.