In the graphic display of quantitative information, various classification schemes are used to organize data for meaningful interpretation. The human mind makes sense of the world by grouping and ordering what the senses perceive. The mind groups elements such as plants, animals, clothing, and so on in processes of qualitative judgment. It orders elements such as big trees, small trees, and so on in processes of quantitative judgment. Grouping and ordering are fundamental to classification schemes used to prepare data for mapping.

A set of data is a selected group of elements—a “population.” This population is first ordered (arranged along the number line from smallest value to largest) and then divided into classes of closely related numeric value. Each class is then represented on the map ...