30. The server of claim 23 , wherein the one or more processors are further configured to derive the one or more negative keywords from the subset of off-topic search criteria by: 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.
23. A server for identifying negative words, comprising one or more processors, a memory coupled to the one or more processors, and a network interface, the one or more processors configured to: receive data encoding an advertisement item and a log comprising a plurality of previously submitted search criteria, wherein each search criterion comprises one or more terms, and wherein when the search criterion was evaluated, the advertisement item was presented in a corresponding search result webpage; evaluate the plurality of previously submitted search criteria to identify one or more negative keywords by: identifying, from the plurality of previously submitted search criteria, a subset of search criteria as being off-topic to the advertisement item, and deriving the one or more negative keywords from the subset of off-topic search criteria; generate one or more files including the one or more negative keywords, the one or more negative keywords prevent the advertisement item from being presented in a search result corresponding to a later submitted search criterion containing at least one of the identified negative keywords; and store the one or more files on computer-readable media.