1. A method of determining a relation between search queries, the method comprising: maintaining a database that associates a search session with at least one search query which has been received from a user terminal during said search session, wherein the database is updated at predetermined time intervals, said database being stored in a memory; determining a total number of search sessions for each user terminal during a first time interval, by referring to said database; determining a first number of search sessions where a first search query is received from said each user terminal during said first time interval, by referring to said database; determining a second number of search sessions where a second search query is received from said each user terminal during said first time interval, by referring to said database; determining a third number of search sessions where both said first search query and said second search query are received from said each user terminal during said first time interval, by referring to said database; calculating conditional probability from comparing said determined first number of search sessions where said first search query is received with said determined third number of search sessions where both said first search query and said second search query are received; calculating correlation by using said total number of search sessions, said first number of search sessions, said second number of search sessions, and said third number of search sessions; and determining a relation between said first search query and said second search query, based, at least in part, upon said calculated conditional probability and said calculated correlation, wherein said steps of calculating conditional probability and calculating correlation are performed by a processor.