15. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a search query including a query term and a substitute term for the query term; receiving data corresponding to a set of documents; accessing a value corresponding to a relevance of the search query to a selected document from the set of documents; determining that the value satisfies a predetermined threshold; and in response to determining that the value satisfies the predetermined threshold, generating a score for the selected document according to a first scoring model, the first scoring model including a first increment value for the substitute term; wherein the first scoring model is different than a second scoring model used to generate scores for one or more other documents from the set of documents, the one or more other documents having values corresponding to the relevance of the search query to each respective other document, where the values of the one or more other documents do not satisfy the predetermined threshold, the second scoring model having a second increment value for the substitute term that is different than the first increment value.
18. The system of claim 15 wherein accessing a value corresponding to a relevance of the search query to a selected document from the set of documents comprises receiving a signal corresponding to an average duration of users' views of the selected document when the query term was used.