The map series above uses the choropleth technique of data visualization. The term is derived from the Greek words CHOROS (place) and PLETH (value). It refers to statistical maps that match distinctive color or shading, representing data, to an enumeration unit which is usually a statistical or administrative area, such as the counties in this series.

It is common to use derived values (rates or ratios) rather than total values because the enumeration units are usually unequal in size and could give misleading impressions if the scheme were based on data totals. The choropleth technique helps the map reader see spatial qualities and variations in the data and is an appropriate tool for creating a sense of an overall geographic pattern. The choropleth technique assumes that the value in the enumeration unit is spread uniformly within the unit.