Sunday, 19 September 2010

Bad Pie Chart

Yes, I know it is an tautology, but under a very small number of circumstances a pie chart can almost work. This one that was published by the California Institute for Telecommunications and Information Technology doesn't work on any level.
As you can see, it is basically 3 pie charts nested one within the other to form concentric rings from which the reader is supposed to be able to make comparisons of changing reading habits between 1960 and 2008.

So right from the start we have an inconsistent timeline with the chart showing a 20 year gap followed by an 18 year gap. This has the effect of preventing any meaningful information being conveyed about the rate of change. If figures are available for 2008, a non-decadal year, then it is reasonable to assume that the author would have been able to locate data for years that would have given a consistent timeline and thus left us, the poor reader with some hope of extracting information from the graphic (assuming that we have a protractor to hand, of course).

This brings us on to a second problem, if this chart is intended to display the changing picture of reading patterns in an undisclosed area, would any one like to postulate how the contribution of Radio has changed between 1960 and 2008? How about is it bigger or less in 2008 than it was in 1960?

In fact, I did get a protractor out, in 1960 the Radio sector was 72 degrees and in 2008 it was 38 degrees so according to this chart it has shrunk by almost half.

this highlights the third problem with this pie chart, because the area taken up by a uniform width band will increase the further it is from the centre of a circle, the area occupied by the 1960 Radio segment is very similar to the area occupied by the segment in 2008. Because people are better at judging area than they are angles the reader is fooled into believing that the segments represent similar populations.

So we are able to reliably extract 4 numbers from this chart without spending time with a printout and a protractor are the three figures for print on the selected years and a figure for computer in 2008 because the author has provided numbers for these. and given that there is no entry for computer prior to 2008 we are left to wonder at the growth rate of this medium, from the chart we could be forgiven for thinking that the computer sprung into being in 2008, but certainly after 1980. this would be something of a shock to those of us using Apple IIs and Sinclairs in the 1970s. So three numbers that are related to the time series, wrapped up in a lot of non-informative ink. This should have been a line chart, or if the data is only available in discreet segments a histogram.


  1. Hi, which program did you use to do the graph?
    Do you know if it can be done using R?

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