|The interest for information
retrieval has existed long before the Internet. The boolean retrieval is
the most simple of these retrieval methods and relies on the use of Boolean
operators. The terms in a query are linked together with AND, OR and NOT
. This method is often used in search
engines on the Internet because it is fast and can therefore be used online.
This method has also its problems. The user has to have some knowledge
to the search topic for the search to be efficient, e.g., a wrong word
in a query could rank a relevant document non relevant. The retrieved documents
are all equally ranked with respect to relevance and the number of retrieved
documents can only be changed by reformulating the query .
The Boolean retrieval has been extended and refined to solve these problems. Expanded term weighting operations make ranking of documents possible, where the terms in the document could be weighted according to their frequency in the document . Boolean information retrieval has been combined with content-based navigation using concept lattices, where shared terms from previously attained documents are used to refine and expand the query . The Boolean operators have been replaced with fuzzy operators . Weighted query expansion using a thesaurus . A model based on fuzzy set theory allows the interpretation of a user query with a linguistic descriptor for each term [8,9].