1、计算平均值Avg
Avg = (a0 + a1 + ......+ an-1) / n
2、计算sigma
sigma = sqrt( ( (a0-avg) ^2 + (a1-avg) ^2 + ..... +(an-1 -avg) ^2 ) / (n-1) )
3、3 sigma = 3* sigma
6 sigma = 6 * sigma