Interval scale (measurement)

An interval scale is a measurement scale where data points are ordered, differences between values are meaningful and equal, but there is no true zero point. This distinguishes it from ratio scales, which possess a meaningful zero.

On an interval scale, you can meaningfully calculate the distance between two measurements. Temperature in Celsius is the classic example: the difference between 10°C and 20°C equals the difference between 20°C and 30°C. However, 0°C doesn't represent the absence of temperature—it's merely a conventional reference point. You cannot say "40°C is twice as hot as 20°C" in any absolute sense.

Interval scales enable certain mathematical operations: addition, subtraction, and computation of means and standard deviations. They're less restrictive than ratio scales but more informative than ordinal scales, where only ranking matters.

Interval scales appear across disciplines: calendar dates, IQ scores, compass bearings, and pH measurements. They're foundational to statistical analysis, enabling parametric tests that assume equal intervals between values.

The distinction between interval and ratio scales matters because it determines which statistical methods are valid and how results should be interpreted.

Related

Ratio scale (measurement), Nominal scale (measurement), Ordinal scale (measurement), Statistical Analysis, Measurement theory, Data types