Red States and Blue States - Map Interpretation

Map Interpretation

There are several problems in creating and interpreting election maps that should be taken into account. Popular vote data is necessarily aggregated at several levels, such as counties and states, which are then colored to show election results. Maps of this type are called choropleth maps, which have several well known problems that can result in interpretation bias. One problem arises when areal units differ in size and significance, as is the case with election maps. These maps give extra visual weight to larger areal units, whether by county or state. This problem is compounded in that the units are not equally significant. A large county or state may have fewer voters than a small one, for example. Some maps attempt to account for this by using cartogram methods, but the resulting distortion can make such maps difficult to read.

Another problem relates to data classification. Election maps often use a two-class color scheme (red and blue), which results in a map that is easy to read but is highly generalized. Some maps use more classes, such as shades of red and blue to indicate the degree of election victory. These maps provide a more detailed picture, but have various problems associated with classification of data. The cartographer must choose how many classes to use and how to break the data into those classes. While there are various techniques available, the choice is essentially arbitrary. The look of a map can vary significantly depending on the classification choices. The choices of color and shading likewise affect the map's appearance. Further, all election maps are subject to the interpretation error known as the ecological fallacy.

Finally, there are problems associated with human perception. Large areas of color appear more saturated than small areas of the same color. A juxtaposition of differing colors and shades can result in contrast misperceptions. For example, due to the simultaneous contrast effect, the Bezold effect, and other factors, an area shaded light red surrounded by areas shaded dark red will appear even lighter. Differing shades of red and blue compound this effect. Cartographers have traditionally limited the number of classes so that it is always clear which class a color shade represents. Some election maps, however, have broken this tradition by simply coloring each areal unit with a red-blue mixture linked to voting ratio data—resulting in an "unclassified choropleth map". These "purple maps" are useful for showing the highly mixed nature of voting, but are extremely difficult to interpret in detail. The lack of clear classes make these purple maps highly prone to the problems of color perception described above. However, there are pros and cons to both classified and unclassified choropleth maps. Each tend to bring out some patterns while obscuring others. All these points should be taken into account when looking at election maps.