(2015), for a [X 1 , Y, X 2 ] triple, our retrieval model treats the context (X 1 , Y ) and each response (X 2 ) as two documents and converts them to their TF-IDF based vectors (Ramos, 2003) to check for similarity.
we first obtain all candidate responses in the training set that are polite, 7 and calculate their TF-IDF vectors.