The injection of renewable energy brings a big uncertainty to the modern power system, which would have a detrimental influence on the frequency stability. The fluctuating frequency from a specific wind farm is difficult to be represented by system average frequency due to uneven inertia distribution in the system. In order to deal with this issue, the coherency identification is applied to separate the system into a few areas and then further research is conducted. In terms of frequency stability analysis, two factors are often concerned: the rate of change of frequency (RoCoF) and frequency nadir/vertex (FN/FV). Therefore, this paper proposes a novel assessment index, i.e. Max Spatial Distribution Ratio of System Frequency Response (MaDSR), suitable for these two factors to evaluate the effects of the coherent results based on a new concept, Area Frequency Response Center (AFRC). To test the effectiveness of the index, slow coherency identification method is selected to divide a modified IEEE 16machine 68 bus with additional installation of wind farms.