1. A method of clustering documents on a computer with a processor and memory, comprising: identifying, with the processor, entities in each of a plurality of documents in a set of documents; obtaining, with a processor, association rules corresponding to an initial set of documents based on co-occurrences of the entities in the initial set of documents, each of the obtained association rules identifying one or more entities that co-occur with other entities in a given document in the initial set of documents with a frequency that meets a threshold; assigning, with the processor, each of the documents in the set of documents to a first set of clusters in the memory, based on the obtained association rules corresponding to the initial set of documents; after assigning each of the documents in the set of documents to the first set of clusters, then for each given cluster in the first set of clusters, deriving with the processor, a plurality of additional association rules based on co-occurrences of the entities within individual documents in the given cluster, each derived additional association rule being specific to the given cluster in the first set of clusters; and assigning, with the processor, newly received documents to clusters in the first set of clusters based on the obtained association rules and the derived plurality of additional association rules.
8. The method of claim 1 and further comprising: clustering documents assigned to the given cluster into sub-clusters based on the obtained association rules and the derived additional association rules.