Reduction of attributes in an information system (IS) is a basic step for IS analysis. The
original rough set model of Pawlak depends on an equivalence relation which is
strongly constraints. This paper aims to use similarity classes and similarity degrees to
obtain a reduction of IS and indicate an approach by using an example from
biochemistry to get a quantitative structure activity relationship (QSAR). Moreover,
signs of each attribute and degrees of memberships are computed to give a decision
by using the degree of similarity. The suggested approach gives an increase in
decision-making and decision accuracy.