The classic IR model is classified to be boolean model, Vector model and probabilistic model. The Boolean model is based on boolean logic in both the documents and the user's query. Retrieval is based on whether or not the documents contain the query terms. Each term is either present (1) or absent (0). The operators are "AND", "OR", and "NOT".
Although it is quite pop in current IR system, it has some drawbacks. For example, IR is based on binary decision criteria with no notion of partial matching; No ranking of the documents is provided; Information need has to be translated into a Boolean expression which most users find awkward.
-- Shu Wang
