Nominal variables: Those qualitative variables whose values or states have no order or rank are called nominal variables. It is only possible to distinguish equality or inequality between two such objects. For example, "marital status" has states of married, single, divorced, or widowed. These states have no numerical significance, and no particular order or rank. The states could be assigned numerical codes however, such as married = 1, single = 2, divorced = 3, and widowed = 4.

Ordinal variables: Those qualitative variables whose states can be arranged in a rank order, but the difference between two distinct values has no significance are ordinal variables. Ordinal variables may also be assigned numerical values. For example, the states very tall, tall, medium, short, and very short can be arranged in order from tallest to shortest and be assigned an arbitrary scale of 5 to 1. However, the difference between successive values does not necessarily have any quantitative meaning.

Binary variable. These variables may assume only one of two values, such as 0 or 1, good or bad, yes or no, high or low.

Interval (metric) variables. These variables take on numeric values and for which equal differences between values have the same significance. For example, real numbers corresponding to temperature or integers corresponding to an amount of money are considered as interval variables.