9. A system, comprising: a data processing apparatus; and a memory storage apparatus in data communication with the data processing apparatus and storing instructions executable by the data processing apparatus and that upon such execution causes the data processing apparatus to perform operations comprising: accessing a plurality of queries; for each query of the plurality of queries: determining 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 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 a total number of unfiltered search operations based on the first numbers of occurrences of the plurality of queries; determining 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 a first value based on a ratio of the first number of occurrences for of the query to the total number of unfiltered search operations; determining a second value based on a ratio of the second number of occurrences for of the query to the total number of filtered search operations; and determining 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.
9. A system, comprising: a data processing apparatus; and a memory storage apparatus in data communication with the data processing apparatus and storing instructions executable by the data processing apparatus and that upon such execution causes the data processing apparatus to perform operations comprising: accessing a plurality of queries; for each query of the plurality of queries: determining 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 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 a total number of unfiltered search operations based on the first numbers of occurrences of the plurality of queries; determining 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 a first value based on a ratio of the first number of occurrences for of the query to the total number of unfiltered search operations; determining a second value based on a ratio of the second number of occurrences for of the query to the total number of filtered search operations; and determining 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. 14. The system of claim 9 , wherein determining the first number of occurrences comprises: determining that, during a search session for a client device, a query was first received for an unfiltered search operation and then received for a filtered search operation, and in response: discounting the first reception of the query for the unfiltered search operation from the first number of occurrences.