Categorical variables: Data such as the industry of a firm or religion of a person, that indicates only membership of an observation in a certain category, with no other mathematical meaning to the data (no preference or order of the categories, no possibility of mathematical operations on the data).

Ordinal data: Data such as rankings of consumer car preferences. The data has only an order, but no sense of the distances between the data points.

Continuous data: Data on a scale running from low to high on a relatively mathematically fine scale, such as prices or perhaps answers to a survey question where there are enough scale answers. Recall that single questions on Likert or semantic differential-type scales are considered debatable especially when they have five answer points (some believe this to be ordinal data). Some researchers believe that 1 to 7 answer scale data has more chance of beginning to be continuous.