The goal of the evaluation function is to assess the quality (Q) of some tentative rule. YAILS uses an evaluation function which relates two properties of a conjunction of conditions : consistency and completeness [8]. The value of quality is obtained by the following weighted sum of these two properties :
This formula is a heuristic one, resulting from experiments and observations made with YAILS in real world domains. The formula weighs two properties according to the value of consistency (which is judged to be more important). Making the weights dependent on consistency is a way of introducing some flexibility into to the formula thus coping with different situations (such as rules covering rare cases or very general rules). Many other possible combinations of these and other properties are possible (see for instance [1,11]), and YAILS is in itself very easily changed in order to use another quality formula.