On possible and necessary inclusion of intuitionistic fuzzy sets

The problem of measuring the degree of inclusion for two intuitionistic fuzzy sets (IF-sets) is considered. Since the nature of IF-sets includes uncertainty expressed both by the membership and nonmembership functions, this very feature should be also regarded in the construction of the subsethood measures. Therefore, we suggest not a single coefficient measuring the grade of subsethood but two indicators that leave some space to the mentioned uncertainty and correspond rather to necessary and possible inclusion. (C) 2010 Elsevier Inc. All rights reserved.