1. A method of comparing digital documents, the method comprising: filtering documents to reduce a total number of documents for fingerprint comparison at least by: performing a comparison of respective total numbers of links respectively included in a pair of documents in the documents and one or more websites pointed to by the respective total numbers of links based in part or in whole upon rates of change comprising a first rate of change and a second rate of change, wherein the first rate of change associated with at least one respective total number of links is slower than the second rate of change associated with contents of at least one document of the pair of documents in the documents, and the comparison is performed without comparing fingerprints of the pair of documents; and determining whether or not to compare the fingerprints of the pair of documents based at least in part upon the rates of change and whether a degree of difference in the respective total numbers of links from the comparison falls within a predetermined range; and in response to determining that the fingerprints should be compared, generating the fingerprints on determining the fingerprints are not existing, and comparing the fingerprints, at a computing system, to determine a similarity value indicating a degree of similarity of the pair of documents being compared, the similarity value expressing degrees of similarity in addition to whether the pair of documents being compared are identical or not identical.