def kurtosis_custom(column, mean, count): return ((count*df_sum(df_pow(column - mean, int(4)))) / df_pow(df_sum(df_pow(column - mean, int(2))),2)) -3
def skewness_custom(column, mean, count): return ((np.sqrt(count) * df_sum(df_pow(column - mean, int(3)))) / df_pow(sqrt(df_sum(df_pow(column - mean, int(2)))),3))
def variance_custom(column, mean, count): return df_sum(df_pow(column - mean, int(2))) / float(count-1)