计算公式

1.标准差（sqrt(sum(Math.pow(x-mean,2))/n)）
int count = 0;
double mean = 0;//平均值
double m2 = 0;//sum（(x-mean)*(x-mean))

double x = ((Number) value).doubleValue(); x-->当前值
count++;  -->统计个数
double delta = x - mean;   
mean = mean + delta / count;
m2 = m2 + delta * (x - mean); -->等价于m2+(currentX - mean)*（currentX-mean）,右边m2等于(beforeX - mean)*（beforeX-mean)的总和

2.方差  sum(Math.pow(x-mean,2))/n)