Sunday, July 27, 2014

An Overstuffed Series of Donut Charts

In his All About Presentations blog on July 24th Vivek Singh blogged about how to Create professional looking charts in 6 steps. He showed how to use PowerPoint to make a very pretty donut chart series like the one shown above, which is taken from an Ericsson publication called Performance Shapes Smartphone Behavior. (I added the light green background).

When I looked at that series, something didn’t seem to add up, and doesn’t. Those four percentages are shown as separate categories, which implies they are exclusive. But, 40% + 25% + 23% + 20% = 108%. That’s bull dung.

Something is wrong with the raw data. Before you plot percentages you need to check that they total to a hundred percent. (You could have the named percentages total to less than a hundred, if you left out a miscellaneous or other category though).

PowerPoint is set up to automatically scale the total from them to fill a circle on a single pie chart or donut chart, so it won’t object to showing something silly like this. If you tried to plot them on a single chart, you might be more likely to check them. When you saw that 25% did not fill 90 degrees, you would ask what is wrong.         

Perhaps there really are people who use a train or bus and they shop while commuting. Or, maybe there are people who grab a sandwich and have dinner while they are either shopping or commuting. The charts should show what is really happening.

This is a much more subtle error than the pie chart with a total of 271% I blogged about last December as ‘tis the season for pies and artistic charts about them.

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