28. The document ranking method of claim 16 , further comprising: generating a community using content scores and contribution scores of a plurality of documents having words associated with a subject.
29. The document ranking method of claim 28 , wherein the community generating further comprises: calculating 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 using a content score and a contribution score of each of the documents, and constructing a sub-graph by retrieving neighboring documents of a document having a greatest document ranking score.