有些不明白方差，求解

alicejingli · 发表于 2010-12-18 22:47:27

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

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

sqxz · 发表于 2010-12-18 23:27:02

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

jxdavidolever · 发表于 2010-12-18 23:29:29

能解释一下LZ说的那句话什么意思么？

有些不明白方差，求解

所属分类: GMAT考试

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