1. A computer-implemented method comprising: identifying a category from a knowledge base, the category including a plurality of entities; identifying a set of queries including terms related to the entities included in the category from a query stream; calculating a specificity value for the category, based at least on the entities included in the category and the set of queries, comprising: determining a probability distribution for co-occurrence of the terms in the set of queries with the entities associated with the category; determining a background probability distribution for the occurrence of the terms in the set of queries; and comparing the probability distribution of the co-occurrence of the terms in the set of queries with the entities associated with the category and the background probability distribution to obtain the specificity value for the category; and classifying the category as a coherent category, wherein a coherent category is a category whose specificity value satisfies a threshold.