Thanks for the thanks.
Being rude about pie charts is all good fun and, for many of us, an
acceptable variant on blood sports. But if precise decoding of
proportions is the aim, then tables beat pie and bar charts hand
down, or the actual numbers can just be added to the graph!
One major objection to pie charts is that their effectiveness breaks
down very rapidly as displays depart only slightly from extreme
simplicity. You need only think about comparing 30 category
proportions across 3 pie charts, or 10 categories across 10 pie
charts, to realise that.
Irrelevantly, but it just sprang to mind: Some of you may enjoy this
algorithm, probably known throughout history, for sharing a pie
(cake, whatever) between two children. One child gets to cut and the
other gets to choose.
Nick [email protected]
Ronan Conroy
As well as a thank-you to Nick for his very useful wrapper to Stata's
rather awkward -pie- command, -pieplot-, I thought I might share an
interesting paper which illustrates the history of this humble graph
and reviews the evidence for its supposedly inferior performance to
the bar chart.
The author concludes:
There seems to be little objective basis for a prejudice against the
pie based on considerations of speed or accuracy of estimation—the
pie chart does as well, if not better, on simple tasks such as the
estimation of a single proportion or the com- parison of a small
number of proportions. On the other hand, the natural competitors of
the pie suffer significant disadvantages. For example, the simple bar
chart does not provide an integrated representation of the whole,
thus making part–whole estimation more difficult. This drawback may be
alleviated by providing a reference bar, but the individual
proportions will be at varying distances from the reference bar. The
divided bar chart does provide a pictorial representation of the
whole but it is less desirable than the pie for exactly the same
reason that instru- ments that have circular dials (speedometers,
altimeters, airspeed indicators, clocks, etc.) are generally
preferred to those that use linear representations—they take up less
space while providing the same or better resolution. Finally, the pie
provides at least five natural anchors (0%, 25%, 50%, 75%, 100%)
compared to only two, or at most three, for the divided bar (0%, 50%,
100%). A bar chart —without a reference bar—affords no natural
anchors to assist in the accurate estimation of proportions
http://www.psych.utoronto.ca/~spence/Spence%202005.pdf
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