Hierarchical Alignment Decomposition Labels for Hiero Grammar Rules

Selecting a set of nonterminals for the synchronous CFGs underlying the hierarchical phrase-based models is usually done on the basis of a monolingual resource (like a syntactic parser). However, a standard bilingual resource like word alignments is itself rich with reordering patterns that, if clustered somehow, might provide labels of different (possibly complementary) nature to monolingual labels. In this paper we explore a first version of this idea based on a hierarchical decomposition of word alignments into recursive tree representations. We identify five clusters of alignment patterns in which the children of a node in a decomposition tree are found and employ these five as nonterminal labels for the Hiero productions. Although this is our first non-optimized instantiation of the idea, our experiments show competitive performance with the Hiero baseline, exemplifying certain merits of this novel approach.