The "levels of measurement", or scales of measure are expressions that typically refer to the theory of scale types developed by the psychologist Stanley Smith Stevens. Stevens proposed his theory in a 1946 Science article titled "On the theory of scales of measurement". In this article Stevens claimed that all measurement  in science was conducted using four different types of scales that he called "nominal", "ordinal", "interval" and "ratio". 

More info on Wikipedia: http://en.wikipedia.org/wiki/Level_of_measurement

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