2. 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 query prefix from a user; obtaining a reference parameter for the user; identifying one or more likely queries that are likely to co-occur with the reference parameter in user activity sessions; obtaining initial ranking scores for the one or more likely queries; computing, for each likely query of the one or more likely queries, a respective new ranking score including multiplying the initial ranking score for the likely query by a ranking factor associated with the likely query, wherein the ranking factor R is given by: R = P ⁡ ( x | q ) P ⁡ ( x ) , wherein P(x|q) is a measure of a likelihood of the likely query x occurring in a user activity session given that the reference parameter q also occurred in a same user activity session, and wherein P(x) is a measure of the likelihood of the likely query x appearing in a user activity session; determining a ranking of the one or more likely queries according to the new ranking scores; and providing the ranking of the one or more likely queries in response to receiving the query prefix.
2. 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 query prefix from a user; obtaining a reference parameter for the user; identifying one or more likely queries that are likely to co-occur with the reference parameter in user activity sessions; obtaining initial ranking scores for the one or more likely queries; computing, for each likely query of the one or more likely queries, a respective new ranking score including multiplying the initial ranking score for the likely query by a ranking factor associated with the likely query, wherein the ranking factor R is given by: R = P ⁡ ( x | q ) P ⁡ ( x ) , wherein P(x|q) is a measure of a likelihood of the likely query x occurring in a user activity session given that the reference parameter q also occurred in a same user activity session, and wherein P(x) is a measure of the likelihood of the likely query x appearing in a user activity session; determining a ranking of the one or more likely queries according to the new ranking scores; and providing the ranking of the one or more likely queries in response to receiving the query prefix. 19. The system of claim 2 , wherein the operations further comprise: determining whether the one or more likely queries occur in a baseline collection of query completions; and filtering any of the one or more likely queries that do not occur in the baseline collection of query completions.