On this basis, the MaxBLEU metric is defined as: where r k• denotes all the references in the kth group.
we begin by calculating all the multi-BLEU scores between each hypothesis and grouped references, and pick the score for the sentence with the highest BLEU as the score for this set of hypothesis, so that we make an alignment between generated hypothesis h to the group-aware references r. For simplicity, one response can only be aligned to one group reference, and multi-group references are not considered in this work.