1. A method of filtering a set of documents considered to be relevant to a subject, comprising: selecting a set of documents determined to be relevant to a subject; clustering, by a processing device, the set of documents into a hierarchy of clusters based on a partitional clustering approach, the cluster hierarchy comprising a tree structure having a plurality of branches, each branch having at least one of a leaf node or a sub tree, wherein the clustering comprises dividing the set of documents into a number of clusters of the hierarchy; flattening, by a processing device, one or more branches of the cluster hierarchy, wherein the flattening comprises recreating a cluster that was divided by combining two or more clusters that resulted from the cluster's division; and re-clustering, by a processing device, documents in the flattened branches based on an agglomerative clustering approach to form a modified cluster hierarchy, wherein when more than one flattened branch is selected for re-clustering, each flattened branch is re-clustered in parallel with other flattened branches, and wherein the modified cluster hierarchy includes clusters formed by partitional clustering and clusters formed by agglomerative clustering.