Most SMT systems, not only phrase-based models (Och and Ney, 2004;Koehn et al., 2003;Xiong et al., 2006), but also syntax-based models (Chiang, 2005;Galley et al., 2006;Huang et al., 2006;Shen et al., 2008), usually extract rules from word aligned corpora.
these systems suffer from a major drawback: they only extract rules from 1-best alignments, which adversely affects the rule sets quality due to alignment mistakes.