1. A computer-implemented method comprising: clustering a plurality of users with respect to one or more latent variables in a probability distribution model of a relationship between a set of users and a set of items, the probability distribution model comprising a probability distribution of the set of items with respect to the latent variables; as new items are added to the set of items, updating the probability distribution of a current set of items with respect to the latent variables including determining counter values incremented in response to item selections in the current set of items and according to user membership in categories represented by the latent variables and where updating the probability distribution includes calculating, for each category, a fraction of counts for an item relative to all the counter values for items in the particular category; and generating a relationship score for one or more users with respect to the set of items, each relationship score relating the particular user to a particular item based on the particular user's category memberships and based on the updated probability distribution of the set of the items with respect to the latent variables.