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.
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. 11. The method of claim 1 , wherein after the re-clustering, the hierarchy includes a first set of clusters generated based on the partitional clustering approach and a second set of clusters generated based on the agglomerative clustering approach, wherein the first set of clusters includes documents that were partitionally clustered but not agglomeratively clustered, and wherein the second set of clusters includes documents that were both partitionally clustered and agglomerative clustered.