@classmethod
    def WGSS(cls, clusters, matrix):
        wgss = 0
        for c in clusters:
            n = len(c.all_elements)
            d = mean(numpy.array(get_intra_cluster_distances( c, matrix))**2) 
            wgss += (n-1)*d
        return wgss*0.5
C-H description of the "Within group sum of squares".
dataset/ETHPy150Open victor-gil-sepulveda/pyProCT/pyproct/clustering/evaluation/metrics/calinskiHarabasz.py/CalinskiHarabaszCalculator.WGSS