Interval Scale Measurement: Definition, Features and Examples

Measure something as 20°C one day and 30°C the next and you know the second day was 10 degrees warmer. But was it 'one and a half times as hot'? No, and the reason is the heart of interval scale measurement. This guide explains what an interval scale is, its key features, everyday examples, how it compares with nominal, ordinal and ratio scales, and how to collect interval-style ratings in a survey.

What is interval scale measurement?

An interval scale is a level of measurement where values are ordered and the gaps between them are equal, but there is no true zero. It is one of four levels of measurement, alongside nominal, ordinal and ratio. The four levels were set out by psychologist S. S. Stevens in his 1946 paper On the Theory of Scales of Measurement in Science.

Because the steps are equal, you can say how far apart two values are. Because zero is arbitrary, you cannot say one value is a multiple of another.

AhaSlides scale slide asking audiences how the room feels, from Too cold to Too hot, on a 1 to 9 scale with equal steps

Key characteristics of interval scales

Equal intervals

The gap between adjacent values is the same everywhere on the scale. The jump from 10°C to 11°C is the same size as the jump from 20°C to 21°C.

Arbitrary zero point

Zero is a convention, not 'nothing'. 0°C is where water freezes, not the absence of temperature. The Fahrenheit scale puts zero somewhere else entirely, and both are valid.

Addition and subtraction make sense

You can calculate differences, means and standard deviations. That is why interval data supports most common statistical tests.

Ratios do not make sense

With no true zero, 40°C is not 'twice as hot' as 20°C. Convert both to Fahrenheit (104°F and 68°F) and the ratio changes, which shows it was never meaningful. A ratio scale such as kelvin, which starts at absolute zero, does support ratio statements.

Ordered values

Every value sits in a clear rank order, so you can say which is higher and by exactly how much.

5 examples of interval scale measurement

1. Temperature in Celsius or Fahrenheit

The textbook case. The difference between 20°C and 30°C equals the difference between 30°C and 40°C, but 0°C is not 'no heat'.

2. IQ scores

IQ tests are standardised so that equal score differences are treated as equal differences in performance, and a score of zero does not mean zero intelligence. Many researchers treat IQ as interval, though some argue it is closer to ordinal.

3. Calendar years

The gap from 1990 to 2000 equals the gap from 2000 to 2010. There is no 'zero' year that means an absence of time.

4. Time of day

On a 12-hour or 24-hour clock, the gap from 1:00 to 2:00 equals the gap from 3:00 to 4:00. Midnight is a convention, not the absence of time.

5. Standardised test scores

Scores on tests such as the SAT or GRE are scaled so that point differences are comparable. A score of zero does not mean 'no knowledge'. Like IQ, they are generally treated as interval.

Interval scales compared with other scales

Scale	What it tells you	Example	True zero?
Nominal	Categories only, no order	Fruit type: apple, banana, cherry	No
Ordinal	Order, but not the gaps	Race positions: 1st, 2nd, 3rd	No
Interval	Order and equal gaps	Temperature in Celsius	No
Ratio	Order, equal gaps and a true zero	Weight in kilograms	Yes

Each level contains the information of the one before it. Ratio scales allow every calculation, interval scales allow differences and averages, ordinal scales allow ranking and medians, and nominal scales allow counting.

Are survey rating scales interval?

This is where interval scales matter most for researchers. A five-point satisfaction rating looks like an interval scale, but the gap between 'satisfied' and 'very satisfied' may not feel equal to the gap between 'neutral' and 'satisfied'. That would make the data ordinal.

In practice, many researchers treat well-designed rating scales with five or more evenly labelled points as approximately interval. Norman (2010) found parametric tests hold up well on this data, while Jamieson (2004) argued that single Likert items should be analysed as ordinal. A sensible approach: label every point clearly, use evenly spaced numeric steps, and report medians and distributions alongside means.

Collect interval-style ratings with AhaSlides

The AhaSlides rating scale lets you set a numeric range with labelled ends, for example 1 to 5 from very dissatisfied to very satisfied. Participants answer on their phones, and the average for each statement appears live. It suits customer satisfaction, employee engagement and audience feedback.

AhaSlides scale slide asking audiences to rate the workshop content, pace and overall experience from Very dissatisfied to Very satisfied on a 1 to 5 scale

For more rating question formats, see our Likert scale examples and 5-point Likert scale guide, or start from the template library.

Create a rating scale

Conclusion

Interval scales give you equal steps and meaningful differences, but no true zero and no ratios. Knowing which level you are working with decides which statistics are valid, so check it before you average, compare or report. When you collect ratings, label every point and keep the steps evenly spaced.

Interval Scale Measurement: Definition, Features and Real-Life Examples