Dominance-based Rough Set Approach (DRSA) is an extension of classical Rough Set Theory (RST). Approximation of class unions is a very important approach in DRSA. Aiming at the disadvantage of the classical method we presented a new methodology for approximation of class unions based on dominance-matrix. It only needs to calculate the dominance-matrix and does not need to consider the preference relations one by one. Thus it greatly simplifies the process. Besides it is intuitive and efficient. The example illustrates its feasibility and efficiency.
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