Ideally, we would like to estimate the parameters of the mapping function so as to directly optimize an automatic MT performance evaluation metric, such as TER or BLEU on the full translation search space.
this is extremely computationally intensive for two reasons: (a) optimizing in the full translation search space requires a new decoding pass for each iteration of optimization; and (b) a direct optimization of TER or BLEU requires the use of a derivative free, slowly converging optimization method such as MERT (Och, 2003), because these objective functions are not differentiable.