Generalizing Hierarchical Phrase-based Translation using Rules with Adjacent Nonterminals

Human Language Technologies: The 2010 Annual Conference of the North American Chapter of the ACL, pages 349–352, Los Angeles, California, June 2010. c⃝2010 Association for Computational Linguistics Generalizing Hierarchical Phrase-based Translation using Rules with Adjacent Nonterminals Hendra Setiawan and Philip Resnik UMIACS Laboratory for Computational Linguistics and Information Processing University of Maryland, College Park, MD 20742, USA hendra, resnik @umd.edu Abstract Hierarchical phrase-based translation (Hiero, (Chiang, 2005)) provides an attractive frame- work within which both short- and long- distance reorderings can be addressed consis- tently and ef�ciently. However, Hiero is gen- erally implemented with a constraint prevent- ing the creation of rules with adjacent nonter- minals, because such rules introduce compu- tational and modeling challenges. We intro- duce methods to address these challenges, and demonstrate that rules with adjacent nontermi- nals can improve Hiero's generalization power and lead to signi�cant performance gains in Chinese-English translation. 1 Introduction Hierarchical phrase-based translation (Hiero, (Chi- ang, 2005)) has proven to be a very useful com- promise between syntactically informed and purely corpus-driven translation. By automatically learn- ing synchronous grammar rules from parallel text, Hiero captures short- and long-distance reorderings consistently and ef�ciently. However, implementa- tions of Hiero generally forbid adjacent nonterminal symbols on the source side of hierarchical rules, a practice we will refer to as the non-adjacent nonter- minals constraint. The main argument against such rules is that they cause the system to produce multi- ple derivations that all lead to the same translation � a form of redundancy known as spurious ambiguity. Spurious ambiguity can lead to drastic reductions in decoding ef�ciency, and the obvious solutions, such as reducing beam width, erode translation quality. In Section 2, we argue that the non-adjacent non- terminals constraints severely limits Hiero's gener- alization power, limiting its coverage of important reordering phenomena. In Section 3, we discuss the challenges that arise in relaxing this constraint. In Section 4 we introduce new methods to address those challenges, and Section 5 validates the ap- proach empirically. Improving Hiero via variations on rule prun- ing and �ltering is well explored, e.g., (Chiang, 2005; Chiang et al., 2008; Zollmann and Venugopal, 2006), to name just a few. These proposals dif- fer from each other mainly in the speci�c linguis- tic knowledge being used, and on which side the constraints are applied. In contrast, we complement previous work by showing that adding rules to Hiero can provide bene�ts if done judiciously. 2 Judicious Use of Adjacent Nonterminals Our motivations largely follow Menezes and Quirk's (2007) discussion of reorderings and generalization. As a speci�c example, we will use a Chinese to En- glish verb phrase (VP) translation (Fig. 1), which represents one of the most prominent phrase con- structions in Chinese. Here the construction of the Chinese VP involves joining a prepositional phrase (PP) and a smaller verbal phrase (VP-A), with the preposition at the beginning as a PP marker. In the translation, the VP-A precedes the PP, a shift from pre-verbal PP in Chinese to post-verbal in English. ���� ���� …

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