1. Nominal: A variable can be treated as nominal when its values represent categories with no intrinsic ranking; for example, the department of the company in which an employee works. Examples of nominal variables include region, zip code, or religious affiliation.

2. Ordinal : A variable can be treated as ordinal when its values represent categories with some intrinsic ranking; for example, levels of service satisfaction from highly dissatisfied to highly satisfied. Examples of ordinal variables include attitude scores representing degree of satisfaction or confidence and preference rating scores.

3. Ratio : A variable can be treated as scale when its values represent ordered categories with a meaningful metric, so that distance comparisons between values are appropriate. Examples of scale variables include age in years and income in thousands of dollars.

4. Interval : ordered, constant scale, but no natural zero , differences make sense, but ratios do not (e.g., 30°-20°=20°-10°, but 20°/10° is not twice as hot! e.g., temperature (C,F), dates