Guest Editorial for the special issue on type-2 fuzzy sets and systems

Type-2 fuzzy sets and systems are more general and potentially more powerful than their type-1 counterparts because they model membership function (MF) uncertainties, which can be very valuable in applications where uncertainties are prevalent. In an interval type-2 fuzzy set (IT2 FS) and fuzzy logic system (FLS), all of the MF uncertainties are uniformly weighted, whereas in a general type-2 fuzzy set (GT2 FS) and FLS, the MF uncertainties are nonuniformly weighted. Since the preparation of the first Special Issue of the IEEE Transactions on Fuzzy Systems that appeared in 2007 on such fuzzy sets and systems, much has happened. Since the 2007 Special Issue, more than 2000 journal and conference articles about T2 FSs and FLSs have appeared; there have been two special sections of the IEEE Computational Intelligence Magazine devoted to them: one in 2007 and the other in 2012; in addition, special or invited sessions about them now appear regularly at all conferences devoted to fuzzy sets and systems. It is therefore very appropriate to have a second Special Issue of these transactions about Type-2 fuzzy sets and systems. For the record, a total of 45 submissions were received in response to the open call. These papers were rigorously evaluated according to the normal reviewing process of the IEEE Transactions on Fuzzy Systems. As in the previous Special Issue, the evaluation process took into consideration factors pertaining to originality, technical quality, presentational quality, and overall contribution. In all, seven papers were accepted for publication. The editors provide here an overview of the technical articles presented in this issue.