Examine the set of data for categories or types.
Check the categories for intrinsic ordering. Ordering means the data can be ordered from highest to lowest. For example, a list of dates can be ordered from earliest to last, but a list of eye colors cannot be ordered. If no ordering exists, the variable is considered categorical or nominal.
If ordering exists, check the data for equal spacing. Data with intrinsic ordering and equal difference between data is called interval variable. Data with ordering and unequal spacing is called ordinal. For example, a list of prices with equal increments (i.e., $7.50, $8.00, $8.50) is considered an interval variable. A list of shirt sizes (i.e., small, medium, large) would be considered ordinal because the differences cannot be equally measured.