1. A method for selectively deleting clusters of conceptually-related words from a probabilistic generative model for textual documents, comprising: receiving a current model, which contains terminal nodes representing random variables for words and contains one or more cluster nodes representing clusters of conceptually related words; wherein nodes in the current model are coupled together by weighted links, so that for a cluster node with an incoming link from a node that has fired which causes the cluster node in the current model to fire with a probability proportionate to a weight of the incoming link, an outgoing link from the cluster node to another node causes the other node to fire with a probability proportionate to the weight of the outgoing link; and processing, at a computer system, a given cluster node in the current model for possible deletion by, determining a number of outgoing links from the given cluster node to terminal nodes and/or cluster nodes in the current model; determining that the determined number of outgoing links is less than a minimum value; and deleting the given cluster node from the current model.