Previous attempts have dealt with the overfitting problem by limiting the maximum phrase length (DeNero et al., 2006;Marcu and Wong, 2002) and by smoothing the phrase probabilities by lexical models on the phrase level (Ferrer and Juan, 2009).
(DeNero et al., 2006) experienced similar over-fitting with short phrases due to the fact that the same word sequence can be segmented in different ways, leading to specific segmentations being learned for specific training sentence pairs.