Nominal scales lack an underlying continuum that lets other scales convert.

Evidence

The key point about nominal scales is that they are not ordered. The big gulf lies between nominal and the other kinds of scales: ordinal, interval, ratio, and fuzzy. For the other kinds there is an underlying conceptual continuum: one can debate and move between the other kinds of scales because of this underlying conceptual continuum. — Gary Goertz, Social Science Concepts and Measurement, 1279

Categorization

• DEFINE: Cleanly separates nominal from ordinal/interval/ratio/fuzzy by the presence or absence of an underlying conceptual continuum.
• SEED: Angle: why the qualitative/quantitative split in methods is really a nominal-vs-ordered split, and what that implies for measurement debates.