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StatClassForm edit

Title Why Not to Use Pie Charts for Communicating Resultshem
Graph Displayed class
Graph Subgroup General Principles
Classification-Graph Type Pie Chart
Code Added No
Date Original
Original Date June 15, 2011
Modified Date

Contributor/Email Mark K Jones, Amadeus Software Ltd (formerly GlaxoSmithKline) and Susan Duke (email: susan.p.duke@gsk.com)
Contributor1 Modification/Email

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Disclaimer The opinions expressed in this document are those of the author and may not represent the opinions of the U.S. Food and Drug Administration or other authors.
Type of Data

Description and Purpose Why not to use a pie chart?
• Computers can easily make pie charts but human brains aren’t particularly good at processing angles (Cleveland)
• Some colors trick the eye into looking larger than other colors in a pie chart
• A pie chart relies on the brain’s use of table-lookup processing, which is inferior to pattern-recognition processing (Cleveland)


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Data

Example1Title Figure 1: Pie Chart
Example1Description Consider the pie chart below. Put the letters A to F in order of the sizes (smallest to largest) of the associated segments in the pie chart below.

Statisticians and statistical programmers were asked this question. Here are the results:

| Number of responses | 38 |
| Number who were completely correct (included letter D and order as requested) | 2 (5%) |
| Number who got the right order but missed D | 18 (47%) |

Example1Image
Click on figure for enlarged image
Example2Title Figure 2: Pie Chart Data in Dotplot
Example2Description *What to use if not a pie chart? Use a dotplot*

This dotplot has exactly the same data as was in the pie chart, A-F, smallest to largest.

If tempted to use a piechart, ask yourself the following questions:
Did you get the order right from the pie chart?

If you did get the order right from the pie chart was it easier (ie, more efficient) to extract the same information from the dot plot below?

Only 53% of people who took this little survey accurately read this pie chart. How many would accurately read the dot plot below?

There were a few different points to be made with this simple example:
Cleveland describes that judging angles is a task the human brain is not particularly proficient with. Some people have a natural aptitude for it, but many will struggle, especially when in this case some of the segments are close in size.

Did you miss letter D? 92% of those who responded in the table above did! This may have been slightly unfair, the letter D wasn't even on the pie chart. However, as you can see from the dot plot D was actually in the data, it just had a value of 0. If it was important that the value for D was 0, then most would have missed it from the pie chart.

If asked to estimate the actual data values from the pie chart, you'd have done so with percentages, as that's all you could have done. The values in the dot plot suggest that this data is not percentages, with the total adding up to much more than 100. It is vital to include units for accurate interpretation of the data.
Example2Image
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Example3Title Figure 3: 3-D Pie Chart
Example3Description While on the topic of pie charts, there is one more point to emphasize. If reading pie charts is a challenge is there something even worse? Yes, indeed, our friend the 3-D pie chart! Take a look at this pie chart (drawn in Excel), again with the same data:

Would you agree that C now looks far bigger than A (which we know it's not), simply because of its prominent position? Please, please, please whatever you do don't use a 3-D pie chart ever!

Of course there are other alternatives for displaying such data. A bar chart would seem appropriate to name just one.
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References The Elements of Graphing Data by William S. Cleveland (Hobart Press, 1994)

Creating More Effective Graphs by Naomi B. Robbins (Wiley, 2005)

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Disclaimer The opinions expressed in this document are those of the author and may not represent the opinions of the U.S. Food and Drug Administration or other authors.
Permission Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF ERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT OLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Reference Image Duke_Piechart_Figure1_200.jpg
Topic revision: r8 - 19 Jun 2012 - 13:41:53 - MaryBanach
 

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