Empirical lower bounds on translation unit error rate for the full class of inversion transduction grammars

Anders Søgaard and Dekai Wu

11th International Conference on Parsing Technology (IWPT 2009)
Paris, France, 7th-9th October, 2009

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Summary

Empirical lower bounds studies in which the frequency of alignment configurations that cannot be induced by a particular formalism is estimated, have been important for the development of syntax-based machine translation formalisms. The formalism that has received most attention has been inversion transduction grammars (ITGs) (Wu, 1997). All previous work on the coverage of ITGs, however, concerns parse failure rates (PFRs) or sentence level coverage, which is not directly related to any of the evaluation measures used in machine translation. Søgaard and Kuhn (2009) induce lower bounds on translation unit error rates (TUERs) for a number of formalisms, incl.~normal form ITGs, but not for the full class of ITGs. Many of the alignment configurations that cannot be induced by normal form ITGs can be induced by unrestricted ITGs, however. This paper estimates the difference and shows that the average reduction in lower bounds on TUER is 2.48 in absolute difference (16.01 in average parse failure rate).

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