A nominal scale is a scale (of measurement) that uses labels to classify cases (measurements) into classes.

The classes must be exhaustive and mutually exclusive. Exhaustive means that all of the cases will have a class in which to be placed. Mutually exclusive means none of the cases will fall into more than one class.

Some examples of variables that use nominal scales would be religious affiliation, sex, the city where you live, etc.

One example of a nominal scale could be "sex". For example, students in a class would fall into two possible classes, male or female. Notice that every student has a class to be placed in (exhaustive), and no student will fall into more than one class (mutually exclusive).

A sample data set is given below left. Notice that it makes no difference if the order of categories is changed, as illustrated below right, since order is irrelevant with nominal data and nominal scales.