1. A method of filtering documents considered to be relevant to a subject, comprising: selecting a set of documents considered to be relevant to a subject, wherein each document is an electronic document that comprise two or more distinct fields including at least a first field and a second field, each of the two or more distinct fields providing data corresponding to a category of data about the respective document; clustering documents in the set of documents into a cluster hierarchy according to data contained in the first field, the cluster hierarchy comprising a plurality of branches, each branch having a sub-tree; merging a received number of levels of the cluster hierarchy; and re-clustering the merged levels according to data contained in the second field.