You've all seen the geospatial maps on television to describe whee the Covid hotspots are, or how the electoral college might shape up, or which states are sweltering in summer heat. But you probably didn't call those maps Choropleths.
Yes, that's right. They have a name! A choropleth map is a thematic map with shaded or patterned areas in proportion to a statistical variable that represents an aggregate summary of a geographic characteristic within each area, such as population density or per-capita income.
Edward Tufte self-published his first book in 1975 since no publisher deemed it valuable. The Visual Display of Quantitative Datawas a landmark book for its day. It had six pages of choropleths, done in grey-scale format, to illustrate geospatial locations for various forms of cancer, by gender. True, they weren’t explained, and in fact, Tufte got them from the CDC, and in the first five printings, the pictures were misaligned. But what images!
I fell in love with the technique, and actually had done a similar thing for the Colorado Air Pollution Commission in 1970. But it took me a long time to figure out how to utilize computer assistance to build maps. The result: the http://www.anywhereanytime.io/covid19 InnovaScapes Institute Covid19 Dashboard.
With that Dashboard, we plot four Covid statistics—Confirmed Cases, Deaths, and each of those aggregate numbers as per capita. We do this for every county in the USA, plus every state, and 225 nations. The Dashboard has data from the first detection in the USA, January 22, 2020.
This posting will deal with ‘reading the maps” four weeks after most states “re-opened” across the country, leading to an undesirable but expected surge in cases—May 29 through June 25, 2020.
Let’s begin with Page 2, the states of the USA. It opens with four maps, as shown below, with a set of bar-graphs at left which show the leading five states for each category, color-coded to match the gradations of the maps. There are two pictures shown—for the first, notice that the left date slider starts at May 29, so all of the Covid19 data prior to that date has been stripped away. For the second, the right date slide is at May 28, so this shows everything up until that date.
Also notice for each set of four maps, there are equivalent bar-graphs to the left of the maps. These show the top five states in each category. Note for example in the first set of maps (Figure A), that California and Texas are each a different color in each of the 4 maps, as are Pennsylvania and Florida. The colors of the bars in the bar-graph correspond to the map colors for any listed state.
Figure A
Similarly to Figure A, Figure B shows California and Texas with four different colors, but New York is always Orange, the top color for these choropleths. Note that all the bar-graph bars are orange, but only a few states on the maps are orange.
Figure B
Next, let's do a side-by-side comparison of the first four months vs. the last four weeks. Figure C is the total number for Confirmed Cases in each state. Note that California is Orange in both; no other state is on both lists, so the "Top Five" have indeed experienced some significant shifting.
Figure C -- early and recent CONFIRMED CASES by state
Figure D does the same as Figure C, except that the Cases are normalized by state populations, which gives a much more fair representation of just how likely you might be to become infected. Here, no state is on both lists, and it is remarkable that the more recent leaders (Arizona, Arkansas, Alabama, South Carolina and Utah) are mostly rural states by comparison with the early leaders (New York, New Jersey, Massachusetts and the District of Columbia). Note also the clear leader, by a factor of two, is Arizona--a state that proudly lauded its independence from government edict or suggestion.
Figure D -- early and recent Confirmed Cases per 100,000 (100K) population
We might do the same for Deaths in each state, and also Deaths per 100K population. Those are shown in Figures E and F. New York was the runaway leader in early deaths--who can forget that tragic fact? The leaders in recent weeks are all at about the same absolute number (Figure E), and in three cases, they are the same states (NY, NJ, MA). CA and TX replaced IL and PA.
Figure E Deaths for the first four months, and for the most recent four weeks
Note that the maps are very similar for the first four months, between Deaths and Deaths per 100K. By contrast, the maps for the more recent four weeks reveal a much lower per capita death rate. How can this be? Well, with CA and TX in the mix, their high death count is still low in per capita terms
Figure F Deaths per capita for the first four months, compared with the most recent four weeks
One more operational note. If you don't find your state listed in the Top Five, how do you find it? And in fact, how do you find data for any particular state? The answer lies in the Selection Table in the upper left hand corner of the main page (Figure 7). This is an alphabetical listing of every state, which can be scrolled by the scroll bar on the right side, or opened up with the Focus Button in the upper right hand corner (see below).
Figure 7 Alphabetic list to begin. Click on any column, and it lists the highest to the lowest, Click again and it inverts the list to list the lowest first. Shown here for Confirmed Cases
The effects of scrolling can be seen by reading down the list. Figure 8 illustrates that California is the clear leader for the most recent four weeks in Confirmed Cases and Deaths, but only 16th in C/100K
Well, a lot of material here, but I hope this gives you a little better idea of how to read choropleths. And, while we're at it, to navigate the Dashboard with a little more ease.
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