1. A computer-implemented method comprising: receiving a user-submitted query; generating an initial ranking of one or more documents responsive to the user-submitted query, the one or more documents including a first document responsive to the user-submitted query; determining that the user-submitted query does not match any record in a model of queries and documents, the model comprising a plurality of records, each record specifying an association between a respective model query, a respective model document, and respective click data aggregated for all users that select the respective model document in response to the respective model query; in response to determining that the user-submitted query does not match any record in the model, forming one or more variants of the user-submitted query to generate one or more other queries that each match a respective record in the model, each of the one or more other queries being different from the user-submitted query; deriving a generalized quality of result statistic for the first document responsive to the user-submitted query from respective data associated in the model with one or more of the other queries, wherein: the respective data for a given other query of the one or more of the other queries is associated with the matching respective record in the model and specifies aggregate user behavior for the first document when the first document was referenced in search results for the given other query, and the user behavior in selection of a reference to the first document in the search results for the given other query; and providing the generalized quality of result statistic as input to a subsequent document ranking process for the user-submitted query that generates, using the generalized quality of result statistic for the first document responsive to the user-submitted query, a subsequent ranking of the one or more documents responsive to the user-submitted query.