19. The computer-readable medium of claim 12 , wherein deriving the one or more negative keywords from the subset of off-topic search criteria comprises: identifying subsets of words in each of the off-topic search criteria; associating each subset of words with an importance metric; and summing the associated importance metric for each occurrence of the subset of words in the off-topic search criteria.
22. The computer-readable medium of claim 19 , wherein deriving the one or more negative keywords from the subset of off-topic search criteria further comprises: assigning the subset of words with the highest summed importance metric as negative keywords.