Rough set model and its eight extensions

As a relatively new computing tool to deal with vagueness and uncertainty, rough set has drawn much attention of many researchers. By now many kinds of extended models of rough set have been proposed and investigated. However, there is no comprehensive survey on extended models of rough set in the literature, this paper attempts to present such a survey according to three expansion ways, i.e. direct extension of the definition of rough set, extension in the certainty environment, and extension in the uncertainty environment. Some relationships between the extended models and the classical model are also given. This article shows the improvement and the development process of rough set theory.