1. One or more computer readable media storing information to enable a computing device to perform a process of predicting a probability that an input out-of-document phrase that is not in a document is relevant to the document, the process comprising: applying an in-document phrase relevance measure to the target document to get a list of top N keywords and respective associated relevance scores; computing a relevance score for the out-of-document phrase by comparing it to the top N in-document keywords based on their relevance scores; and using a regression model to predict a probability of the relevance score of the out-of-document phrase and probabilities of the top N keywords, where the predicting is based on the relevance scores of the top N keywords.