1. A computer-implemented method comprising: receiving resource that includes a set of words; identifying a set of candidate clusters from a probabilistic model that are classified as likely to be active in generating the set of words; generating a vector that characterizes the resource, wherein, for each cluster of the set of candidate clusters, a component of the vector indicates a degree to which the cluster was active in generating the set of words; and using the vector in performing an operation related to the resource.