11. The document ranking system of claim 1 , further comprising: a community generating unit to generate a community based on content scores and contribution scores of a plurality of documents having words associated with a subject.
11. The document ranking system of claim 1 , further comprising: a community generating unit to generate a community based on content scores and contribution scores of a plurality of documents having words associated with a subject. 12. The document ranking system of claim 11 , wherein the community generating unit is configured: to calculate a sum of document ranking scores of documents with respect to the words associated with the subject, the document ranking score of each document being determined based on a content score and a contribution score of each of the documents; and to retrieve neighboring documents of a document having a greatest document ranking score to construct a sub-graph.