Final answer:
The statement 'typically, we have two kinds of variables: discrete and continuous' is true about variables. Discrete variables result from counting, and continuous variables result from measuring, while nominal variables are used for categorical data.
Step-by-step explanation:
The true statement about variables among the options provided is that typically, we have two kinds of variables: discrete and continuous. Nominal variables, which represent categorical data, can be expressed using numbers as labels, but those numbers do not have mathematical value. Therefore, the statement that nominal variables are expressed by numbers is misleading without further context. A variable must vary, by definition, so the statement that a variable does not vary is incorrect. Lastly, categorical variables can be considered a type of discrete variable because they represent distinct categories or groups.
Moreover, it's important to understand that variables are characteristics of interest in a population and can assume different types of values. Variables such as the number of points earned by a student (quantitative) and a person's party affiliation (categorical) are exemplary of discrete and nominal variables respectively. In contrast, continuous variables involve measurements, like the height of a person or the temperature on a given day, which can take on an infinite number of values within a range.