A former student asked me about this chart from the New York Times that highlights much higher prices of hospital procedures in the U.S. relative to a comparison group of seven countries.
The dot plot is clearly thought through. It is not a default chart that pops out of software.
Based on its design, we surmise that the designer has the following intentions:
- The names of the medical procedures are printed to be read, thus the long text is placed horizontally.
- The actual price is not as important as the relative price, expressed as an index with the U.S. price at 100%. These reference values are printed in glaring red, unignorable.
- Notwithstanding the above point, the actual price is still of secondary importance, and the values are provided as a supplement to the row labels. Getting to the actual prices in the comparison countries requires further effort, and a calculator.
- The primary comparison is between the U.S. and the rest of the world (or the group of seven countries included). It is less important to distinguish specific countries in the comparison group, and thus the non-U.S. dots are given pastels that take some effort to differentiate.
- Probably due to reader feedback, the font size is subject to a minimum so that some labels are split into two lines to prevent the text from dominating the plotting region.
In the Trifecta Checkup view of the world, there is no single best design. The best design depends on the intended message and what’s in the available data.
To illustate this, I will present a few variants of the above design, and discuss how these alternative designs reflect the designer's intentions.
Note that in all my charts, I expressed the relative price in terms of discounts, which is the mirror image of premiums. Instead of saying Country A's price is 80% of the U.S. price, I prefer to say Country A's price is a 20% saving (or discount) off the U.S. price.
First up is the following chart that emphasizes countries instead of hospital procedures:
This chart encourages readers to draw conclusions such as "Hospital prices are 60-80 percent cheaper in Holland relative to the U.S." But it is more taxing to compare the cost of a specific procedure across countries.
The indexing strategy already creates a barrier to understanding relative costs of a specific procedure. For example, the value for angioplasty in Australia is about 55% and in Switzerland, about 75%. The difference 75%-55% is meaningless because both numbers are relative savings from the U.S. baseline. Comparing Australia and Switzerland requires a ratio (0.75/0.55 = 1.36): Australia's prices are 36% above Swiss prices, or alternatively, Swiss prices are a 64% 26% discount off Australia's prices.
The following design takes it even further, excluding details of individual procedures:
For some readers, less is more. It’s even easier to get a rough estimate of how much cheaper prices are in the comparison countries, for now, except for two “outliers”, the chart does not display individual values.
The widths of these bars reveal that in some countries, the amount of savings depends on the specific procedures.
The bar design releases the designer from a horizontal orientation. The country labels are shorter and can be placed at the bottom in a vertical design:
It's not that one design is obviously superior to the others. Each version does some things better. A good designer recognizes the strengths and weaknesses of each design, and selects one to fulfil his/her intentions.
P.S. [1/3/20] Corrected a computation, explained in Ken's comment.