17. A method for determining proximity to ad topics, comprising: identifying, with one or more processors, one or more character strings associated with a topic of a content item; identifying, with one or more processors, one or more search queries, each search query having one or more query terms that are associated with keywords of the content item, the keywords used in determining whether or not to embed the content item in Web pages including search results responsive to the search queries; determining, with one or more processors, one or more metrics of the search queries based, at least in part, on the one or more character strings, the metrics indicating proximity of the query terms of each search query to the topic of the content item; and determining, with one or more processors, whether a subset of the search queries is irrelevant to the topic in accordance with the one or more proximity metrics, where a search query is irrelevant to the topic when the query terms of the search query are not proximate to the topic.
17. A method for determining proximity to ad topics, comprising: identifying, with one or more processors, one or more character strings associated with a topic of a content item; identifying, with one or more processors, one or more search queries, each search query having one or more query terms that are associated with keywords of the content item, the keywords used in determining whether or not to embed the content item in Web pages including search results responsive to the search queries; determining, with one or more processors, one or more metrics of the search queries based, at least in part, on the one or more character strings, the metrics indicating proximity of the query terms of each search query to the topic of the content item; and determining, with one or more processors, whether a subset of the search queries is irrelevant to the topic in accordance with the one or more proximity metrics, where a search query is irrelevant to the topic when the query terms of the search query are not proximate to the topic. 20. The method of claim 17 , wherein the one or more metrics are determined using a Bayesian filter.