Quand on dispose de connaissances a priori sur les sorties possibles d’un problème d’étiquetage, il semble souhaitable d’inclure cette information lors de l’apprentissage pour simplifier la tâche de modélisation et accélérer les traitements. Pourtant, même lorsque ces contraintes sont correctes et utiles au décodage, leur utilisation lors de l’apprentissage peut dégrader sévèrement les performances. Dans cet article, nous étudions ce paradoxe et montrons que le manque de contraste induit par les connaissances entraîne une forme de sous-apprentissage qu’il est cependant possible de limiter.

In Statistical Machine Translation (SMT), the constraints on word reorderings have a great impact on the set of potential translations that are explored. Notwithstanding computationnal issues, the reordering space of a SMT system needs to be designed with great care: if a larger search space is likely to yield better translations, it may also lead to more decoding errors, because of the added ambiguity and the interaction with the pruning strategy. In this paper, we study this trade-off using a state-of-the art translation system, where all reorderings are represented in a word lattice prior to decoding. This allows us to directly explore and compare different reordering spaces. We study in detail a rule-based preordering system, varying the length or number of rules, the tagset used, as well as contrasting with oracle settings and purely combinatorial subsets of permutations. We focus on two language pairs: English-French, a close language pair and English-German, known to be a more challenging reordering pair.