Therefore, we use the inverse frequency of a response in a conversation corpus to indicate its specificity level.
we first build the response collection R by extracting all the responses from D. For a response Y ∈ R, let f Y denote its corpus frequency in R, we compute its Inverse Response Frequency (IRF) as: where |R| denotes the size of the response collection R. Next, we use the min-max normalization method (Jain et al., 2005) to obtain the NIRF value.