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 'psychometrics'.]
See the original article:
Stevens, S. S. (1946). On the theory of scales of measurement. Science, 103(2684), 677-680. [PDF].
For modern criticism, a good place to begin is:
Velleman, P. F., & Wilkinson, L. Nominal, ordinal, interval, and ratio typologies are misleading. [PDF]
