落在一个标准偏差之外的概率是1/3,两个标准差之外的概率是1/20,三个标准差之外是百分之一
One standard deviation happens one draw out of three, two standard deviations one out of twenty, three standard deviations is one out of one hundred.
正态母体特征值（母体均值、百分位值、百分率、标准差和变异系数等）的检验是工程中常见的问题。
The normal population characteristics, such as the mean, standard deviation, percentile, percentage and coefficient of variation, are often tested in engineering.
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