Friday, March 24, 2017

An ineffective pie chart – in a slide deck on Delivering Effective Presentations

Recently one of my Google searches for images led me to an ineffective pie chart similar to the one shown above. The original was the eighth slide from a deck on Delivering Effective Presentations that was posted at Slideshare by Terri L. Jensen on May 18, 2014. Both the content and form are abysmal.

The data came from a web page titled Fear/Phobia Statistics at Statistic Brain, and the data is crap as I discussed in a blog post on July 1, 2012 titled A bogus list of top ten phobias. On December 7, 2014 I blogged about how Statistic Brain is just a statistical medicine show.

The form is awful because those three percentages add up to 172.5%, as shown above. They should have been shown via a bar chart, not a pie chart. Microsoft Excel unfortunately will let you produce a pie chart like this. (The wedges it shows are rescaled by dividing by whatever the total is. 74% gets shown as 42.9%, 68% gets shown as 39.4%, and 30.5% gets shown as 17.7%). An article by Nathan Yau at FlowingData titled How to Spot Visualization Lies cautions:
“Some charts specifically show parts of a whole. When the parts add up to more than the whole, this is a problem. For example, pie charts represent 100 percent of something. Wedges that add up to more than that? Peculiar.”

Pie charts also are not very effective for comparing similar percentages. It is hard to see the 3.5% difference between Public Speaking (42.9%) and Death (39.4%) until you extend the line between them upward, as shown above.

With a horizontal bar chart showing the actual percentages it is easier to see the difference between 74% and 68%.   

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