Scales used to express measurements. Usually ratio, interval, ordinal or nominal types of scales are recognized. Dichotomous and count types are sometimes seen as their own. Other classifications exist. [For "scale" as a sum of individual items in psychometry - see also tag 'scale construction'.]

See the original article:

S. S. Stevens, On the Theory of Scales of Measurement. http://gaius.fpce.uc.pt/niips/novoplano/mip1/mip1_201314/scales/Stevens_1946.pdf

For modern criticism, a good place to begin is

Paul Velleman and Leland Wilkinson, Nominal, Ordinal, Interval, and Ratio Typologies are Misleading. http://www.cs.uic.edu/~wilkinson/Publications/stevens.pdf

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