Measurement Scale

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Within psychology and many disciplines that draw from psychology, variables are commonly classified as being in one of four types:

Scale type Example Most informative way
can be compared
Nominal Variable Are you male or female? Do they differ?
Ordinal Variable Are you: Sad, Neither Sad nor Happy or Happy? Which is higher?
Interval-Scale Variable Please rate your happiness on a scale of 1 to 10, where 1 means very sad and 10 means happy. Which is higher and by how much
Ratio-Scale Variable How many hugs have you received today? Ratios

This classification originates from attempts to draw precise relationships between how data was collected and how it can be analyzed.[1] Although the usage of the terms is widespread in the social sciences, this distinction is not considered fundamental in applied survey analysis (e.g., it is routine in commercial research to compute means from ordinal data and the distinction between interval and ratio-scale variables is largely irrelevant in statistical analysis, except to the point that it influences decisions regarding how to transform variables.


  1. Stevens, S. S. (1959). Measurement. In C. W. Churchman, ed., Measurement: Definitions and Theories, pp. 18-36. New York: Wiley. Reprinted in G. M. Maranell, ed., (1974) Scaling: A Sourcebook for Behavioral Scientists. Chicago, Aldine: 22-41.