17. The system of claim 11 , wherein one of the one or more watch time signals is a document-query fraction score that is based on a portion of a total k of the watch times associated with the query that are attributable to the first resource.
11. 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: identifying, by a processor of a computer in the one or more computers, one or more sessions for a query, each session comprising a chain of respective resources and linked to each other and watched by a respective user, each session beginning with a first resource that was identified by a first search result responsive to the query linked to one or more second resources, wherein each second resource was associated with a different resource in the session by a respective link, and wherein the user visited each second resource by following the links; associating a total of watch times of the respective resources watched in the sessions with the query; calculating one or more watch time signals for the first resource and the query based on the total of watch times associated with the query; after the one or more sessions have ended: receiving the query from a user; obtaining a search result responsive to the query, wherein the search result identifies the first resource and has an associated score S; calculating an updated score S′ based on at least S and a watch time function, the watch time function being a function of the one or more watch time signals; and providing the updated score S′ to a process for ranking search results including the first search result.