The idea of rough set was proposed by Zdzislaw Pawlak as a new mathematical tool to deal with vague concepts. The idea of the rough set consists of the approximation of a set by a pair of sets, called the lower and the upper approximation of the set (Pawlak, 1982). In fact these approximations are interior and closure operations in a certain topology generated by available data about elements of the set. In other words, the rough set approach is based on knowledge of an agent (or group of agents) about some reality and its ability to discern some phenomena, processes, objetcs, etc. Thus the approach is based on the ability to classify data obtained from observation, measurements, etc.