Most statistical models are described in terms of one-dimensional variables. These may either be:
Categorical – taking values in a finite set.
Ordinal – taking values in an ordered set.
Discrete – taking values in the integers.
Continuous – taking values in the real range.
Minor variation on these sets are possible, e.g. the natural numbers. Multi-dimensional variables take values in products of these sets.
In this course we confine attention to categorical, discrete and continuous variables. Sometimes categorical (or qualitative) variables are called factors. However we reserve factor as a technical term related to a model formula.
Exercise 1.2
Give examples of these types of variables.