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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: The not so humble pie |

Date |
Wed, 21 Oct 2009 17:44:06 +0100 |

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 n.j.cox@durham.ac.uk 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 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: RE: The not so humble pie***From:*David Bell <dcbell@iupui.edu>

**Re: st: RE: The not so humble pie***From:*Jeph Herrin <junk@spandrel.net>

**References**:**st: The not so humble pie***From:*Ronan Conroy <rconroy@rcsi.ie>

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