1. A method for generating a reference set for use during document review, using one or more processors, comprising: obtaining a collection of unclassified documents; identifying one or more features within each of the unclassified documents; generating clusters of the features and selecting at least one feature from one or more of the clusters as reference set candidates; assigning a classification code to each reference set candidate; and refining the reference set candidates, comprising: grouping the reference set candidates into further clusters; selecting at least one of the reference set candidates in one or more of the further clusters as the further reference set candidates; assigning a classification code to the further reference set candidates; and forming a reference set from the unclassified documents associated with the further classified reference set candidates; and wherein all the steps are performed by a suitably programmed computer.