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A Terrible Pie Chart

I saw this graphic reblogged by NPR on Tumblr (originally posted by Luminous Enchiladas, though I can't be sure of the creator), and I must say that it is impressive.

Olympics vs Mars

Olympics vs Mars

There are some pretty substantial problems with this impressively bad graphic.

  • Pie charts should only be used when comparing parts to a whole. The $17.5 billion dollars that went to the Olympics and the Curiosity Rover wasn't a priori some whole amount of money. Treating it as "the whole" implies that there was only $17.5 billion dollars from wherever to be spent, and that it was spent only on the Olympics and Mars.
  • The pieces of the pie chart aren't labeled with the dollar amounts. Instead, the pieces are labeled with the piece's name which does address a complaint with pie charts (namely that the reader needs to continually look back and forth from the chart to the key). Because there are only two pieces, there is room for including the dollar figures in the chart area. With more complicated charts, this wouldn't be the case.
  • This chart uses an unnecessary "3D" effect which obscures the true areas being compared. A flat pie chart would be less misleading.

Additionally, there are some general problems with pie charts which make them inferior to other charts (specifically bar charts):

  • Comparing areas is difficult. Cleveland (1985) writes about how area comparisons are subject to bias, and Schmid (1983) specifically describes how, when comparing two circles (e.g. two pie charts of different size used to indicate change over time), the area of the larger circle is underestimated relative to the smaller.
  • Comparing angles is difficult. Cleveland (1985) states that ordering the sections of a pie chart is prone to error based on earlier empirical research.

References:

  • Cleveland, W. S. (1985). The elements of graphing data. Monterey, Calif: Wadsworth Advanced Books and Software.
  • Schmid, C. F. (1983). Statistical graphics: Design principles and practices. New York: Wiley.

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