The arithmatic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively. What value is exactly 2 standard deviations less than the mean?
A 10.5
B 11.0
C 11.5
D 12.0
E 12.5
什么是 2 standard deviations less than the mean? 怎么求，谢谢高手赐教！
好像是A 查了一下定义 The central limit theorem says that the distribution of a sum of many independent, identically distributed random variables tends towards the normal distribution. If a data distribution is approximately normal then about 68% of the values are within 1 standard deviation of the mean (mathematically, μ ± σ, where μ is the arithmetic mean), about 95% of the values are within two standard deviations (μ ± 2σ), and about 99.7% lie within 3 standard deviations (μ ± 3σ). This is known as the 68-95-99.7 rule, or the empirical rule.
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发表于 2009-4-19 18:06:00
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