**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]

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