The control performance standard（CPS） is currently widely used as an area control performance assessment criterion in electric power grids of China. It is noted that， when it was introduced， the assessment criterion rules and some parameters have been altered， including mainly short period averaging of CPS1 and relinquishment of direct use of CPS2 and penalty return， etc. Proceeding from theoretical basis and quantitative calculation， the difference between North America and China in using the CPS is analyzed and compared as has the possible impact on the final assessment. Some relevant concepts involved in the control performance assessment are clarified.