有些不明白方差，求解

weixiushiwansui

If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than2.5 standard deviations from the mean?


(A) 12.0

(B) 13.5

(C) 17.0

(D) 23.5

(E) 26.5

---------------------------------------------------------------------------------------------------------------------------------
【答案】 A
我想问values is more than2.5 standard deviations from the mean 这句话是啥意思，

season5924

能解释一下LZ说的那句话什么意思么？

sunflower88

ONE standard deviation from the mean:  within 3 of the mean 平均值加减一倍方差

TWO ....: within 2*3 of the mean平均值加减两倍方差

2.5*3=7.5