We can describe the scales of measurement used in psychological research in terms of their abstract number properties. The measures that we use in psychology vary in how they assign numbers to psychological characteristics. All assign numbers, but the properties of the abstract number system that are represented by those numbers can vary widely.

The following pages have a series of examples drawn from psychological research that illustrate each of the four primary scales of measurement.

• nominal
• ordinal
• interval
• ratio

In addition, we will include a separate discussion of Likert-type scales as they are treated as both ordinal and interval scales in psychological research.