1. A computer-implemented method performed by data processing apparatus comprising one or more computers in data communication, the method comprising: receiving queries, each query submitted for a respective first search operation; for each query: initiating the first search operation and receiving data indicating first resources identified by the first search operation as being responsive the query, each first resource having a corresponding score by which the first resource can be ranked in responsiveness to the query relative to other first resources; determining a search probability ratio for the query, the search probability ratio being a measure of a likelihood of the query being submitted for a second search operation, the second search operation being of search operation type that is different from a search operation type of the first search operation; initiating the second search operation and receiving data indicating second resources identified by the second search operation as being responsive the query, each second resource having a corresponding score by which the second resource can be ranked in responsiveness to the query relative to other second resources; determining, for a second resource identified by the second search operation, a first resource identified by the first search operation that is descriptive of a second resource; determining, based on the search probability ratio, the corresponding score of the first resource, and the corresponding score of the second resource, whether to insert a search result identifying the second resource in a set of search results identifying the first resources; and for each query for which a determination is made to insert a search result identifying the second resource in a set of search results identifying the first resources, inserting the search result identifying the second resource in the set of search results identifying the first resources.