18. The computer-readable medium of claim 17 , the relevance score includes an average logarithmic odds that a term of the associated search criterion occurs in a set of keyword clusters associated with a topic of the advertisement item.
17. The computer-readable medium of claim 16 , wherein identifying the subset of search criteria as being off-topic to the advertisement item comprises: associating, with each of the plurality of search criteria, a relevance score, and if the relevance score is under a threshold relevance value, assigning the associated search criterion as being off-topic.