Who Ate My Slice? [Chart Review]
This isn’t your typical blog post bashing pie-charts. The post here is an assessment of a chart that literally doesn’t add up. A while back, I noticed something similarly wrong with the numbers in a different chart. If you add up all of the numbers in the pie chart below, you don’t get to 100 or 100%. This is the whole (no pun intended) point of a pie chart; parts of a whole.
[source]
Below, I added up the six slices of the pie and only came to 99 or 99%. Am I the only one who does this for fun? For whatever reason, I can’t pass up on doing these checks and math when seeing a chart like the one above. Now, I’m sure the reason for the missing 1% is rounding. Instead of 41%, I’m sure the actual number is 41.4% and so on. Whenever making data visualizations, be sure to check the details and do the simple math to ensure everything adds up correctly. Something as simple as the pie chart slices not adding up to 100% could lead to questioning the competency of the creator.
Is this being too critical or do you think the minor details matter?
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April 13th, 2009 at 7:27 am
Tony -
Usually for people like you (uh, us) there is a disclaimer, “Numbers may not add to 100 due to rounding”.
There is a real problem with Pie Chart Rounding in Excel. If you show percentage labels with no decimal percentages, the percentages are forced to sum to 100, and if the rounded whole-number percentages don’t add up, Excel arbitrarily adjusts the displayed percentages. This can be almost meaningless, showing 33-33-34 for equal thirds, but for more intricate data sets, similar values can be adjusted in a strange and wondrous manner, for example, percentages of 21-21-21-13-8-8-8 (which add to 100, by the way) being displayed as 22-20-21-13-8-8-8 in 2003 and as 23-21-21-13-8-7-7 in 2007.
To solve the problem use a number format of 0.0% instead of 0%. Apparently the designers of Excel decided that people who can understand decimal percentages don’t require that the pie wedges add to exactly 100.
Of course, there are cases where the inputs to a pie chart do not add to 100%, as described in Pie Chart Plotting Deficiency. Instead of including a wedge for “Other 10%”, they left it off. But the chart looks pretty.
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April 13th, 2009 at 7:50 am
Tony -
What ’s the font of the numbers in the chart ? I wanna install it .
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April 13th, 2009 at 11:47 am
Jon – I thought I remembered those posts when I saw your comment. I agree that we can avoid this issue by using the format 0.0%. Your second example of the missing slice is unbelievable. Definitely a reason to go with the bar chart.
Liu – I didn’t create the chart and listed the source as from BusinessWeek. You may be able to find out by contacting them directly. I think the default font – arial, enlarged and bold looks pretty good.
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