4. Interval scales of measurement
Because the size between intervals is equal within any one scale, you can add and subtract these values. Multiplication and division is not permitted because there is no absolute zero, and, for instance, a temperature of 40°C is not twice as hot as a temperature of 20°C.
Comparisons across different scales (for example centigrade to fahrenheit) should be handled with care, because they’re not starting from the same zero point and although the intervals within a scale are equal, the size of the intervals on different scales will vary.