where the symbol rank = , as we mentioned previously, means that the right-hand side is rank equivalent to the left-hand side (i.e., we can ignore the normalizing constant P(Q)), P(D) is the prior probability of a document, and P(Q|D) is the query likelihood given the document. In most cases, P(D) is assumed to be uniform (the same for all documents), and so will not affect the ranking. Models that assign non-uniform prior probabilities based on, for example, document date or document length can be useful in some applications, but we will make the simpler uniform assumption here. Given that assumption, the retrieval model specifies ranking documents by P(Q|D), which we calculate using the unigram language model for the document