There has been a good deal of work on information retrieval systems that have continuous weights assigned to the index terms that describe the records in the database, and1 or to the query terms that describe the user queries. Recent articles have analyzed retrieval systems with continuous weights of either type andlor with a Boolean structure for the queries. They have also suggested criteria which such systems ought to satisfy and record evaluation mechanisms which partially satisfy these criteria. We offer a more careful analysis, based on a generalization of the discrete weights. We also look at the weights from an entirely different approach involving thresholds, and we generate an improved evaluation mechanism which seems to fulfill a larger subset of the desired criteria than previous mechanisms. This new mechanism allows the user to attach a "threshold" to the query term.
Statement of the Problem
A. Boolean and Fuzzy SystemsA set of documents D* exists for retrieval. A set of index terms I* also exists. In a traditional retrieval system, a function F exists mapping D* X I* into [0,1] . For d ED* and T E I*,A Boolean query Q can now be constructed from the terms in I* and the operations AND, OR, and NOT. Given a query Q , a retrieval status value (RSV) can be obtained for each document d by using the usual truth table methods for the operators. The RSV is 1 (true) if d is "about" Q and 0 (false) if d is not "about" Q. A strictly Boolean retrieval system will retrieve all the documents for which the RSV is Looked at in another way, a term T defines a set T*, the 1 PI. set of documents "about" T :
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