1. A keyphrase extraction system comprising: a probability computation component that calculates probability values of a joint probability of a candidate term and a document, a marginal probability of the candidate term and a marginal probability of the document; a partial mutual information metric computation component that computes a partial mutual information metric for the candidate terms based on the probability values; and, a summarization component that identifies one or more summary keyphrases based, at least in part, upon the partial mutual information metric.