1. Dept. of Economic Management, North China Electric Power Univ., Hebei 071003, P.R. China.
2. Professor, Dept. of Industrial and Systems Engineering, North Carolina State Univ., Raleigh, NC 27695.
3. Professor, Dept. of Economic Management, North China Electric Power Univ., Hebei 071003, P.R. China.
4. Professor, School of Economics and Management, North China Electric Power Univ., Beijing 102206, P.R. China (corresponding author).