1. A computer-implemented method comprising: obtaining a list of documents from a document collection that contain a first phrase, the first phrase being relevant to a query; for each document in the list: determining whether the document includes one or more related phrases of the first phrase, where each related phrase has an actual co-occurrence rate of the related phrase and the first phrase in the document collection that exceeds an expected co-occurrence rate of the related phrase and the first phrase in the document collection; ranking the documents in the list based on a quantity of related phrases determined for each document, so that documents with more related phrases are ranked higher than documents with fewer related phrases; and selecting at least some of the highest-ranked documents to include in a result to the query.