14. The computer-readable storage medium of claim 12 , wherein the minimum number of sources is three sources.
12. A non-transitory computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method for selecting links while updating a probabilistic generative model for textual documents, the method comprising: receiving a current model, which contains terminal nodes representing words and cluster nodes representing clusters of conceptually related words, wherein nodes in the current model are coupled together by weighted links, wherein if a node fires, a link from the node to another node is activated and causes the other node to fire with a probability proportionate to the weight of the link; applying a set of training documents containing words to the current model to produce a new model, and while doing so, determining expected counts for activations of links and prospective links, determining link-ratings for the links and the prospective links based on the expected counts, and selecting links to be included in the new model based on the determined link-ratings; and making the new model the current model.