Interval Scale

The level of measurement of a variable in mathematics and statistics is a classification that was proposed in order to describe the nature of information contained within numbers assigned to objects and, therefore, within the variable. The levels were proposed by Stanley Smith Stevens in his 1946 article On the theory of scales of measurement. According to Stevens' theory of scales, different mathematical operations on variables are possible, depending on the level at which a variable is measured.
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in research,a scale where equal intervals in the scale correspond to equal changes in the characteristic being measured, eg temperature scales (Celsius/Fahrenheit). Differences between objects can be analysed statistically (which is not possible with nominal or ordinal scales).

This scale of measurement allows you to not only rank order the items that are measured, but also to quantify and compare the sizes of differences between them (no absolute zero is required).
See also, Measurement scales .