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Statistical Translation After Source Reordering: Oracles, Context-Aware Models, and Empirical Analysis

Published online by Cambridge University Press:  14 May 2012

MAXIM KHALILOV  and
KHALIL SIMA'AN
MAXIM KHALILOV
Affiliation:
Institute for Logic, Language and Computation, University of AmsterdamP.O. Box 94242, 1090 GE Amsterdam, The Netherlands e-mails: maxim@tauslabs.com, k.simaan@uva.nl
KHALIL SIMA'AN
Affiliation:
Institute for Logic, Language and Computation, University of AmsterdamP.O. Box 94242, 1090 GE Amsterdam, The Netherlands e-mails: maxim@tauslabs.com, k.simaan@uva.nl
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Abstract

In source reordering the order of the source words is permuted to minimize word order differences with the target sentence and then fed to a translation model. Earlier work highlights the benefits of resolving long-distance reorderings as a pre-processing step to standard phrase-based models. However, the potential performance improvement of source reordering and its impact on the components of the subsequent translation model remain unexplored. In this paper we study both aspects of source reordering. We set up idealized source reordering (oracle) models with/without syntax and present our own syntax-driven model of source reordering. The latter is a statistical model of inversion transduction grammar (ITG)-like tree transductions manipulating a syntactic parse and working with novel conditional reordering parameters. Having set up the models, we report translation experiments showing significant improvement on three language pairs, and contribute an extensive analysis of the impact of source reordering (both oracle and model) on the translation model regarding the quality of its input, phrase-table, and output. Our experiments show that oracle source reordering has untapped potential in improving translation system output. Besides solving difficult reorderings, we find that source reordering creates more monotone parallel training data at the back-end, leading to significantly larger phrase tables with higher coverage of phrase types in unseen data. Unfortunately, this nice property does not carry over to tree-constrained source reordering. Our analysis shows that, from the string-level perspective, tree-constrained reordering might selectively permute word order, leading to larger phrase tables but without increase in phrase coverage in unseen data.

Type
Articles
Information
Natural Language Engineering , Volume 18 , Issue 4 , October 2012 , pp. 491 - 519
Copyright
Copyright © Cambridge University Press 2012

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