However, due to the severe sparsity of short text posts, we modify the calculation of commonly-used topic coherence measure based on word co-occurrences in messages tagged with the same hashtag, named as hashtag-document, assuming that those messages discuss related topics 8 .
we calculate the coherence score of a topic given the top N words ranked by likelihood as below: where w k i represents the i-th word in topic k ranked by p(w|k), D(w k i , w k j ) refers to the count of hashtag-documents where word w k i and w k j cooccur, and D(w k i ) denotes the number of hashtagdocuments that contain word w k i .