While some variants have been considered (Och, 2003;Flanigan et al., 2013), we use an expected BLEU approximation, assuming hypotheses are drawn from a log-linear distribution according to their parameter values (Smith and Eisner, 2006).
we assume the probability of a translation candidate e s,m is proportional to (exp (w h s,m )) µ , where w are the parameters being optimized, h s,m is the vector of the features for e s,m , and µ is a scaling parameter.