Applied Abstraction - the qualitative / quantitive dichotomy

Nominal - numbers are either the same or not

Ordinal - if different, one number is bigger than the other

Interval - differences in numbers can be compared

Ratio - zero is a significant point

The properties described are nested - nominality is needed for ordinality, which in turn is needed for interval-ality, in turn needed for ratio-ality.

Anything interval level and above is considered quantitative. Most qualitative measures serve only nominal function, i.e. whether something is the same or not.

More formally: http://en.wikipedia.org/wiki/Level_of_measurement [HT: Soo Mei]