As a general tendency, the performance of all approaches worsens as sentence length increases.
we believe this choice provides an optimal trade-off between fully manual error analysis (Farrús Cabeceran et al., 2010;Popović et al., 2013;Daems et al., 2014;Neubig et al., 2015), which is very costly and complex, and fully automatic error analysis (Popović and Ney, 2011;Irvine et al., 2013), which is noisy and biased towards one or few arbitrary reference translations.