Reader Jeff G. sent us to this post from Floating Sheep, which walks through an analysis showing which states have the highest beer consumption in the United States. Jeff is not amused by several of their maps.
The first one utilizes overlapping bubbles, which is generally a bad idea but especially bad when the data is as dense as depicted here:
This is a great example to illustrate why the default use of maps for geographical data is sometimes misplaced. The greatest feature of this map (and many others) is the scarcity of data in the middle and the density around major city centers. This just tells us about the overall population density!
When we plot data on maps, we usually want to highlight something other than population density.
The second map, called the "beer belly of America", has circulated a bit on the Web. This is a case of throwing out too much data. It appears that the original data set contains the number of times bars and grocery stores were searched by location of Google Map users (two numbers per location). The plotted data consists only of whether bars or grocery stores were searched more, thus one bit (binary datum) per location.
Because of excessive data reduction, it appears that most of the country is a vast expanse of yellow. I'm sure if one goes back to the frequency of bar or grocery store searches, one will find that yellow comes in many shades.
While the maps are quite ugly, I like the way the website walks through their analysis process. I would say that their maps are less intended for final presentation as they are intended to aid exploration during the analysis. Indeed, at one point, they computed the number of bars per 10,000 residents (starting with North Dakota, 6.54), which is really the best way to summarize this information. It would be interesting to see this data plotted at the state level.
One technical note: the "beer belly" map contains a hidden assumption that the distribution of Google Map searches is the same as the distribution of population. If not, what we are looking at is the combined effect of the popularity of Google Map searches and beer consumption.