The similarity is calculated as the reverse of the harmonic mean of ranks as sim(a i , a j ) = 1 2 ( 1 r j + 1 r i ), where r j and r i refer to the rank of the a j and a i when a i and a j are given as queries, respectively.
prior work on translation-based Q&A retrieval does not recognize this issue and uses the translation model as it is; essentially no attention seems to have been paid to improving the performance of the translation-based approach by enhancing the quality of translation models.