Classed Choropleth Maps

Choropleth maps portray areal data using areal divisions which are often boundaries defined by units such as counties or states. A Classed Choropleth Map shows data combined into a smaller number of groups which is then portrayed in intervals. The choice of the number of groups affects the resulting map greating. Some formal classsification techniques include equal step classification, classifying by quantiles, natural break classing, and minimum variance classification.

The above choropleth map is classified using equal steps classification. The total data range is first divided into categories chosen by the cartographer. A drawback to this technique is that there may be empty categories or categories with many members, as is the case here, since only one state falls into the highest category while many states fall into the second to lowest category.

http://www.ilstu.edu/~jrcarter/Geo204/Choro/map-AmFact-4eqint-orange-80pct.jpg