S. Cronen-Townsend, Y. Zhou and W. Croft. Predicting query performance. In Proceedings of the 25th annual international ACM SIGIR conference on Research and development in information retrieval. Pages: 299 - 306. Tampere, Finland. 2002.
The clarity score measures the Kullback-Leibler divergence of the generation probability of a term from the query from a collection model.