2.1Variables and Data
Variable：某物或某人的某一特征和其他个体不同。
quantitative variables：定量变量either discrete （可以被数）or continuous.（A continuous variable is a variable whose possible values form some interval of
Numbers）Typically, a continuous variable involves a measurement of something, such
as the height of a person, the weight of a newborn baby, or the length of time a car
battery lasts
即离散变量是一群数出来的数，而连续变量是一群来源于测量的数，来源不同。
qualitative variables, also called categorical variables：定性变量（不能计算的变量）
Each individual piece of data is called an observation, and the collection of all
observations for a particular variable is called a data set.