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● A categorical variable is one for which the measurement scale consists of a set of categories.
● Categorical variables for which levels (categories) do not have a natural ordering are called nominal.  列名型变量
● Categorical variables that do have a natural ordering of their levels are called ordinal. 序列型变量
● An interval variable is one that has numerical distances between any two levels of the scale.1   区间型变量
● A binary variable is one takes only two values such as “1” and “0”,   “M” and “F”, etc. 二值型变量