There are four measurement scales (or types of data) to categorize different types of variables: nominal, ordinal, interval and ratio.

Ordinal data has not only the properties of nominal data but also “>” and “<”. With ordinal data, ordering of values is possible, yet the difference between each one is not really known. For example, education level belongs to ordinal variable. We can tell undergraduate degree is higher than high school degree, yet we cannot tell the difference between them.