Optimizing word alignment combination for phrase table training

Combining word alignments trained in two translation directions has mostly relied on heuristics that are not directly motivated by intended applications. We propose a novel method that performs combination as an optimization process. Our algorithm explicitly maximizes the effectiveness function with greedy search for phrase table training or synchronized grammar extraction. Experimental results show that the proposed method leads to significantly better translation quality than existing methods. Analysis suggests that this simple approach is able to maintain accuracy while maximizing coverage. © 2009 ACL and AFNLP.