Three kinds of data.

The heights of Martians and Venusians are known asinterval databecause heights are measured on a scale with constant intervals, in this case, centimetres. For interval data, the absolute difference between two values can always be determined by subtraction.

There are other kinds of data, such gender, state of birth, or whether or not a person has a certain disease, that are not measured on an interval scale. These variables are examples ofnominal or categorical data, in which individuals are classified into two or more mutually exclusive and exhaustive categories. In every case, it is possible to categorise each individual into one and only one category. In addition, there is no arithmetic relationship or even ordering between the categories.

Ordinal datafall between interval and nominal data. Like nominal data, ordinal data fill into categories, but there is an inherent ordering (or ranking) of the categories. Level of heath (excellent, very good, good, fair, or poor) is a common example of a variable measured on an ordinal scale. The different values have a natural order, but the differences or "distances" between adjoining values on an ordinal scale are not necessarily the same and may not even be comparable.