Diagrams That Changed the World

At BBC News, Professor Marcus du Sautoy of the University of Oxford writes about diagrams that have substantially changed the way people look at the world or processed information. Pictured above is one created by Florence Nightingale, depicting fatalities among British forces from April 1854 through March 1855 during the Crimean War:

Although better known for her contributions to nursing, her greatest achievements were mathematical. She was the first to use the idea of a pie chart to represent data.

Nightingale had discovered that the majority of deaths in the Crimea were due to poor sanitation rather than casualties in battle. She wanted to persuade government of the need for better hygiene in hospitals.

She realised though that just looking at the numbers was unlikely to impress ministers. But once those numbers were translated into a picture - her Diagram of the Causes of Mortality in the Army in the East - the message could not be ignored. A good diagram, Nightingale discovered, is certainly worth 1,000 numbers.

On the chart, blue areas represent deaths by preventable diseases, red areas represent deaths by wounds, and black areas represent deaths by other causes.

Link via The Presurfer | Image: Dynamic Diagrams

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The blue area does not exactly represent deaths by preventable diseases. It is the area from the center to the edge of the blue. It is NOT a stacked chart, they regions are overlapping. You see this in November, when the black area is less than the red. She doesn't know how to represent this, so she simply makes the boundary line.

Overall, it was the first graph of its type, so for that it is interesting, but now that we know so much more, it isn't such a great example as some make it seem.
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Yeah this data would be much clearer if she used a stacked bar chart. Her odd pie chart means that the area/radius issues make really understanding the plot very difficult. My management would never let me brief a general with a plot like that.
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The problem with this depiction is the area each "death" is represented by on the page. A regular bar graph would give an equal spatial deption of each death, whereas this diagram creates a larger visual impression for those deaths on the outer perimeter of the pie chart. Nightingale maximizes her argument by putting the blue numbers on the outermost edge.

Unless, of course, she's assigned a set area to each death (e.g. 0.5cm^2), this is a great way to mislead people. Marketers make use of tricks like these all the time.
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