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.
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. 20. The computer-readable medium of claim 19 , wherein the importance metric is determined by multiplying a confidence number by an amount of impressions associated with a set of keyword clusters that in turn are associated with the advertisement item, the confidence number representing a confidence that the off-topic search criterion is not relevant to a set of keyword clusters associated with a topic of the advertisement item.