Abstract
Discussed is the direct and inverse problem of comparison of two linguistic quantities specified as fuzzy sets (relations) in the same universe of discourse. The proposed way of comparison makes use of some logical properties of connectives and implication operators stemming from the foundations of fuzzy sets. A list of plausible equality indices is provided. The inverse problem is formulated; a set of its solutions is provided. Direct impacts of the discussed approach on development of fuzzy modelling are clearly indicated. Also, it is pointed out how fuzzy sets of higher type (or interval-valued sets) can be generated as a result of analysis of performance of any fuzzy model.