This file prepares YOU for the 2nd link in the chain "6 degrees of separation", namely, SETS. I'll discuss the three scales of measurement: NOMINAL, TYPE, ORDER and why and how we need and use these measures. Then, I can proceed to a file which uses SETS to explicate these measures.

In 1940, Harvard psychophysicist S. S. Stevens initiated the theory of measurement scales defining a scale according to The transformation group conserving the scale and the scale's ALLOWABLE STATISTICS. Stevens introduced TYPE and ORDER. The definition of NOMINAL is mine.

MEASUREMENT SCALES TABLE

SCALE	PROTOTYPE	TRANSFORMATION GROUP	ALLOWABLE STATISTICS	EXAMPLES
Nominal ("ID")	Personal name	CONSTANT	Mode(s)	Proper names; technical terms
Typological	Family name	Identity function	Mode(s)	Taxonomy of plants and animals; etiology of diseases
Ordinal	Preference	Isotonic	Median; range	Moh mineral hardness scale; IQ; Richter-Kanamori earthquake scale

See how SETS can GRAPHICALLY CONSTRUCT the MEASURES of NAMING, TYPING, ORDERING.