1. A computer-implemented method for generating a reference set via clustering, comprising: obtaining a collection of unclassified documents; grouping the unclassified documents into clusters; selecting n-documents from each cluster and combining the selected n-documents as reference set candidates, wherein one of the n-documents from each cluster is located closest to a center of that cluster; assigning a classification code to each of the reference set candidates; and grouping two or more of the reference set candidates as a reference set of classified documents.