In 1946, the psychologist Stanley Smith Stevens wrote an article called “On the Theory of Scales and Measurement.”2 In it he describes different scales of measurement, including “nominal” and “ordinal.” Nominal measurements are simply “set membership” statements, such as whether a fetus is male or female, or whether you have a particular medical condition. In nominal scales, there is no implicit order or sense of relative size. A thing is simply in one of the possible sets. Ordinal scales, however, allow us to say one value is “more” than another, but not by how much. Examples of this are the
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