Did prices go up or down? Depends on how one looks at the data
Hanging things on your charts

One of the most frequently produced maps is also one of the worst

Summer is here, many Americans are putting the pandemic in their rear-view mirrors, and gas prices are soaring. Business Insider told the story using this map:


What do we want to learn about gas prices this summer?

Which region has the highest / lowest prices?

How much higher / lower than the national average are the regional prices?

How much has prices risen, compared to last year, or compared to the last few weeks?


How much work did you have to do to get answers to those questions from the above map?

Unfortunately, this type of map continues to dominate the popular press. It merely delivers a geography lesson and not much else. Its dominant feature tells readers how to classify the 50 states into regions. Its color encodes no data.

Not surprisingly, this map fails the self-sufficiency test (link). The entire dataset is printed on the map, and if those numbers were removed, we would be left with a map of the regions of the U.S. The graphical elements of the chart are not doing much work.


In the following chart, I used the map as a color legend. Also, an additional plot shows each region's price level against the national average.


One can certainly ditch the map altogether, which makes having seven colors unnecessary. To address other questions, just stack on other charts, for example, showing the price increase versus last year.


_trifectacheckup_imageFrom a Trifecta Checkup perspective, we find that the trouble starts with the Q corner. There are several important questions not addressed by the graphic. In the D corner, no context is provided to interpret the data. Are these prices abnormal? How do they compare to the national average or to a year ago? In the V corner, the chart takes too much effort to comprehend a basic fact, such as which region has the highest average price.

For more on the Trifecta Checkup, see this guide.



Feed You can follow this conversation by subscribing to the comment feed for this post.


Well, I understand what you're saying, but the map does tell me whether Arkansas (or a number of other states), for example, is in the Gulf Coast region or the Midwest region, in case I'm considering going that direction. I don't get that information from your charts. So, it depends a little on whether that's important to the audience.

At that point, though, it seems the data is pretty averaged. Is ND really the same ballpark price as TN? That might be more important to me to have the data by state than by region, if I'm planning travel to a particular state.


I've been mapping some incidence of coronavirus data and have been dealing with the colours problem. Safest seems to be carrying shades of a colour, So pale green to dark green represent the levels. I've also used blue, green, yellow, orange and red for ascending case rates and that seems OK. Something I haven't tried is using say blue for below average from full blue through to white, and for above average from white to full red. So white is average or median. Main problem is I would need to do a custom palette. I also need to use a logarithmic scale, which makes life a bit more difficult.

Hopefully we will see some research into what is most easily interpreted, although people are still using pie charts when they shouldn't, so I'm not that optimistic. Local newspaper uses software that defaults to a light to dark display which isn't too bad.

Merle: One of teh unfortunate truths about statistics is that we usually need to work with the data we have. If we get regions, we use regions. Sometimes using too small an area also means too much noise, which can be a real pain with maps.

The comments to this entry are closed.