11. A method comprising: calculating, by a computer processor, similarity among a plurality of documents; identifying, in response to a change made to a first document, a second document having a similarity greater than or equal to a first criterion as compared to the first document before the change; notifying a user associated with the second document that the first document has been changed, in response to the change to the first document; acquiring necessity information indicating whether one or more other documents need to be changed due to the change to the first document; and notifying the user associated with the second document of the necessity information.
11. A method comprising: calculating, by a computer processor, similarity among a plurality of documents; identifying, in response to a change made to a first document, a second document having a similarity greater than or equal to a first criterion as compared to the first document before the change; notifying a user associated with the second document that the first document has been changed, in response to the change to the first document; acquiring necessity information indicating whether one or more other documents need to be changed due to the change to the first document; and notifying the user associated with the second document of the necessity information. 12. The method of claim 11 , further comprising: grouping together into a group two or more documents, among the plurality of documents, having a similarity greater than or equal to the first criterion; wherein the group generation section is configured to maintain the first document as a document in the group even when, in response to a change made to the first document, a similarity of the first document to one or more other documents in the group to which the first document belongs is less than a first criterion.