This paper describes a study of the pat- terns of translational equivalence exhib- ited by a variety of bitexts. The study found that the complexity of these pat- terns in every bitext was higher than sug- gested in the literature. These findings shed new light on why “syntactic” con- straints have not helped to improve statis- tical translation models, including finite- state phrase-based models, tree-to-string models, and tree-to-tree models. The paper also presents evidence that inver- sion transduction grammars cannot gen- erate some translational equivalence rela- tions, even in relatively simple real bi- texts in syntactically similar languages with rigid word order.

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