6.1Introducing Normally Distributed Variables
Why the word “normal”? Because, in the last half of the nineteenth century,researchers discovered that it is quite usual, or “normal,” for a variable to have a distribution shaped like that in Fig. 6.1
normally distributed population：处于正态分布下的群体
approximately a normal distribution：If avariable’s distribution is shaped roughly like a normal curve, we say that the variable is an approximately normally distributed variable or that it has approximately a normal distribution（处于近似正态分布下的群体）
A normal distribution is symmetric about and centered at the mean of the variable,
and its spread depends on the standard deviation of the variable—the larger the standard deviation, the flflatter and more spread out is the distribution.
close to the horizontal axis outside the range from μ − 3σ to μ + 3σ
Standardizing a Normally Distributed Variable
实数a,b的转化：
