What is the difference between interval and ratio data? I know that ratio data has a true 0 point. Temperature is an interval data but doesn’t it also have a true 0 point?

For most scales of measurement, especially where physical objects or values are concerned, zero represents a final end point or a situation of ‘nothing to measure’. Take for example, a study which seeks to test the veracity of whether an experimental drug actually does increase height. Human height, in this study, would be one of the outcomes to be measured.

Height (a ratio scale which also includes measurements such as distance walked, degrees of range of motion etc.) have predictable and equal intervals between values on the measuring scale e.g. between 1m and 2m on a meter rule is a value of 1m and between 5 degrees and 10 degrees on a goniometer is 5 degrees etc. However, zero represents the end point of measurement. For instance, it would appear odd in our aforementioned study, for the researcher to list the height of one the participants of the study as 0m (zero meters) – or even more bizarrely, -1.5m tall. These kind of figures tend to invalidate studies of this type because human height can not be measured as a zero value or in negative values. (-5km walked or 0 degrees of elbow flexion appear equally odd). Ratio scales are so called because the measurements are an estimation of the ratio between a magnitude of a continuous quantity and a unit magnitude of the same kind.

Temperature on the other hand, is one of the best known interval scales. The intervals between values on the measuring scale are also known and equal and even though it has a zero point, the zero point does not represent the end point of measurement i.e. it is not a ‘true’ zero point. For example, in a study to compare the effects of cryotherapy and thermotherapy on hamstring flexibility, it would not be out of place to have cryotherapy data captured as 0 degrees Celsius or even -10, -20 or -30 degrees Celsius as zero degrees and negative values are valid on the Celsius and Fahrenheit temperature scales and do not invalidate the data or study.

However, kindly note that temperature measurements on the Kelvin temperature scale is a ratio scale because it has a unique, non-arbitrary zero point called ‘absolute zero’ beyond which no further temperature measurements can be taken.

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