1. A computer-implemented method, comprising: accessing a plurality of queries; for each query of the plurality of queries: determining, by a data processing apparatus, a first number of occurrences of the query for which the query was received for unfiltered search operations, each unfiltered search operation being a search operation for which search results for resources including content of a first type can be returned in response to the query; determining, by the data processing apparatus, a second number of occurrences of the query for which the query was received for filtered search operations, each filtered search operation being a search operation for which search results for resources including the content of the first type cannot be returned in response to the query; determining, by the data processing apparatus, a total number of unfiltered search operations based on the first numbers of occurrences of the plurality of queries; determining, by the data processing apparatus, a total number of filtered search operations based on the second numbers of occurrences of the plurality of queries; for each query of the plurality of queries: determining, by the data processing apparatus, a first value based on a ratio of the first number of occurrences of the query to the total number of unfiltered search operations; determining, by the data processing apparatus, a second value based on a ratio of the second number of occurrences of the query to the total number of filtered search operations; and determining, by the data processing apparatus, a first content type value based on a ratio of the first value to the second value; for each query of the plurality of queries having a first content type value that meets a first threshold, classifying the query as a first content type seeking query, the classification being indicative of likely user interest in content of the first type; and for each query of the plurality of queries having a first content type value that does not meet the first threshold, classifying the query as a first content type independent query, the classification being indicative of unlikely user interest of content of the first type.