6 4 Optimizing Syntactic N-grams N-gram based metrics such as BLEU (Papineni et al., 2002) are still predominantly used to optimize the log-linear parameters of SMT systems, and (to a lesser extent) to evaluate the final translation systems.
n-gram metrics are not well suited to measure fluency phenomena with string-level gaps, and there is a danger that BLEU underestimates the modelling power of dependency language models, resulting in a suboptimal assignment of loglinear weights.