We cast tuning as a ranking problem (Chen et al., 2009), where the explicit goal is to learn to correctly rank candidate translations.
we follow the pairwise approach to ranking (Herbrich et al., 1999;Freund et al., 2003;Burges et al., 2005;Cao et al., 2007), in which the ranking problem is reduced to the binary classification task of deciding between candidate translation pairs.