In statistics, one usually differentiates between
- nominal scales (e.g. man/woman),
- ordinal scales (elementary school/middle school/high school),
- interval scales (degrees Celsius) and
- ratio scales (human height, degrees Kelvin).
Only ratio scales may be multiplied and divided. Interval scales may be subtracted and added. Ordinal scales may only be compared (less/greater/equal). Nominal scales may only be compared in terms of (in)equality.