gallery: tb maps

The display of information can make a marked difference on how data is perceived. Consider the example below of maps which plot the incidence of tuberculosis by county in Illinois:
 

In the map above, the saturation (intensity of color) corresponds to the rate (cases per thousand people) of tuberculosis infections by county. Where are the problem areas?	In the map above, the hue (color, from green to red) corresponds to the rate (cases per thousand people) of tuberculosis infections; the saturation (intensity of color) corresponds to the population of the county. Now where are the problem areas?

This difference occurs because the downstate counties have such a low population that one person with TB is enough to make the rate comparable to the enormously populous Cook County. It can thus become necessary to show two pieces of information: the rate, and how "important" that rate is; this is done on the right by using two independent visual axes. You'll notice that the counties in the south of Illinois are still red, corresponding to the high rate, but washed out due to the low population of the counties.
 
It brings up interesting questions: if you were going to petition for a tax levy to support TB prevention and treatment programs, which map would you use? What does the map on the right imply for people in rural areas, where medical care is already harder to come by, yet where the per-capita TB rate is similar to metropolitan areas? If you were an elected official in southern Illinois trying to make a decision, would it be the same regardless of the map you were shown?
 
Data sources: County boundaries from US Department of Commerce, Economics and Statistics Administration. TB data from IL Department of Public Health. Population data from US Census Bureau. Saturations are scaled logarithmically to account for human visual perception. For a more detailed description of the methods used to color the maps, or for larger images, please mail me.