Methods based on word strings (e.g., BLEU (Papineni et al., 2002), NIST(NIST, 2002), METEOR (Banerjee and Lavie., 2005), ROUGE-L (Lin and Och, 2004), and IMPACT(Echizen-ya and Araki, 2007)) calculate matching scores using only common words between MT outputs and references from bilingual humans.
these methods cannot determine the correct word correspondences sufficiently because they fail to focus solely on phrase correspondences.