Definition
A model of information retrieval (IR) selects and ranks the relevant documents with respect to a user’s query. The texts of the documents and the queries are represented in the same way, so that document selection and ranking can be formalized by a matching function that returns a retrieval status value (RSV) for each document in the collection. Most of the IR systems represent document contents by a set of descriptors, called terms, belonging to a vocabulary V.
An IR model defines the query-document matching function according to four main approaches:
The estimation of the probability of user’s relevance rel for each document d and query q with respect to a set Rq of training documents
$$ \mathrm{Prob}0.24em \left( rel|\mathbf{d},\mathbf{q},{R}_{\mathbf{q}}\right) $$The computation of a similarity function between queries and documents in a vector space
$$...