1. A computer implemented method 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.