5. PROBABILISTIC INFORMATION RETRIEVAL (PIR)
Both Boolean Model and Vector Space Model provide similarity ratings without considering a level of certainty for the output relevance. There are several models based on probability theory that aim to determine the probability of a document being relevant to a query [8].
The Probabilistic retrieval was first proposed by Manor and Kuhns in 1960, [11] and many variants of the Probabilistic Model have been proposed. The user information is represents by a set of possibly weighted keywords given by the users or induced by the system.