What are the three levels of measurement?

The three levels of measurement are nominal, ordinal, and interval-ratio variables.

Nominal scale:

Some variables classify the subjects of a study according to certain categories, which neither occur in a natural order, nor do they take any ‘quantitative’ numerical values. These variables are measured on the nominal scale of measurement.

For example, the name of a person, their hair color, marital status, etc. are all measured on the nominal scale. Note that, even if a variable takes numerical values, it might still be nominal. The number written on the jersey of a volleyball player does not quantify anything, although it is a number; it is simply a symbol to identify the player. This is a numerical nominal scale variable.

Ordinal scale:

Some variables classify the subjects of a study according to certain categories, which occur in a natural order. These variables are measured on the ordinal scale of measurement.

For example, the educational level of a person, classified as- Illiterate, High School Graduate, College Graduate, Post-Graduate, etc., although is a non-numeric variable, there is a natural ordering present. A College Graduate can definitely be placed at a higher position with respect to their education level, when compared to a High School Graduate. This is an ordinal scale variable.