To evaluate the responses generated by all compared methods, we compute the following automatic metrics on our test set:

BLEU: BLEU-n measures the average n-gram precision on a set of reference responses. We report BLEU-n with n=1,2,3,4.

Distinct-1 & distinct-2 BIBREF5: We count the numbers of distinct uni-grams and bi-grams in the generated responses and divide the numbers by the total number of generated uni-grams and bi-grams in the test set. These metrics can be regarded as an automatic metric to evaluate the diversity of the responses. 
 Question: What automatic metrics are used?
BLEU Distinct-1 & distinct-2