January 9, 2008, 4:06 pm
Nightingale’s Rose
by Henry Woodbury
Filed under: Information Design, Visual Explanation
Two ways of reading the word area — its general vs. its mathematical meaning — leads to confusion in this otherwise superb article on Charts in the Economist. The chart in question is Florence Nightingale’s “Diagram of the Causes of Mortality in the Army of the East.” The data is plotted by month in 30-degree wedges. In each month, red represents deaths by injury, blue death by disease, and black death by other causes:
The Economist explains how to interpret the diagram:
As with today’s pie charts, the area of each wedge is proportional to the figure it stands for, but it is the radius of each slice (the distance from the common centre to the outer edge) rather than the angle that is altered to achieve this.
Herein lies the confusion. In fact, the areas of the wedges are not proportional. The data actually maps to the radius of each wedge. It appears that in her annotation, Nightingale used the word area in the generic sense of section or range. The great sweep of blue around the center of each chart is an artifact of the unusual radial plot. Perhaps unintentionally, Nightingale overdramatized the facts that made her case.
Our Creative Director, Piotr Kaczmarek, recalibrated Nightingale’s chart to correct this error. The diagram below uses all the elements of the original, but makes the data proportional to area:
Nightingale’s diagram, often referred to as Nightingale’s Rose or Nightingale’s Coxcomb, represents one of the inherent risks in visual explanation. An image may be so visually interesting — so iconic (a rose, a coxcomb) — that we assume its conclusions without examining its data.
This is better: a stacked bar chart that introduces a scale (!), more readable labels, and a single chart for the entire 1854-1856 period. These changes provide context and continuity, and make clear the two campaigns of the war:
UPDATE: In the comments, hstern recommends separating the three data types to allow better comparison. As it happens, Piotr created that chart as one of his alternatives to the radial plot. I’ve uploaded it below.
Comments
Thanks for the clarification. I came upon your critique while surfing for sites that might support a workshop on spatial thinking and visual learning I am conducting next week. I had already encountered the article from The Economist, and in the absence of Nightengale’s data, I had doubted whether area was truly being represented. You affirmed my suspicion, provided a corrected version and then provided a more appropriate modern version (albeit one not destined to become quite so famous). I wonder if the modern bar graph you provide could have its clarity improved. The type you use in which all three categories are proportionally represented on a single bar for each month would be most appropriate if the total number of deaths per month was the figure of primary interest. However, it is the comparison between the three categories of death that matters most and using three separate bars for each month would lend both a clear comparison and much greater quantitative clarity for each category. On the other hand, since Nightengale’s purpose was primarily persuasion rather than presenting data simply for the sake of knowledge, by showing no separation between your bars and thereby running the colors together, the resultant massive area of blue compared to the other colors becomes quite persuasive.
Posted by hstern on January 12, 2008 at 4:02 am
hstern, excellent points. As it happens, Piotr prepared several alternative bar charts, including one that corresponds to your recommendation. I didn’t want to overburden the initial post with all of them, but in response to your comment I think an update is appropriate.
Posted by Henry Woodbury on January 14, 2008 at 12:00 pm
How about a line chart instead of the final bar chart?
Posted by Hadley on November 11, 2008 at 11:14 pm
Thanks for these notes; the graphics are beautifully constructed. We came across your work whilst developing material of a similar theme for our project on understanding uncertainty. See http://understandinguncertainty.org/ . The data for ‘Diagram of the Causes of Mortality in the Army in the East’ was published by Nightingale in ‘A contribution to the sanitary history of the British army during the late war with Russia’ (1859). This data can be found on a table at http://understandinguncertainty.org/node/214 . We used this data to replicate Nightingale’s graphs, and the resulting charts resembled her original chart and the chart shown in the Economist article. So we believe that Nightingale’s original diagram is correct. Please describe the source of your data that makes you think otherwise!
Posted by Ian on November 20, 2008 at 12:07 pm