When comparing data, which is what a pie chart is for, people have a hard time judging the angles and areas of the multiple pie slices in order to calculate how much bigger one slice is than the others.
Pie Charts Don't Work
A slice of pie is good for serving up a portion of desert. It's not good for making a judgement about how big the slice is, what percentage of 100 it is, or how it compares to other slices.
People have trouble comparing angles and areas to each other. Controlled studies show that people will overestimate the percentage that a pie slice area represents. This is because we have trouble calculating the area based on the space between the two angles that define the slice.
This picture shows how a pie chart is useless in determing the largest value when you have to compare pie slices.
You can't compare angles and slice areas to each other. Human perception and cognition is poor when viewing angles and areas and trying to make a mental comparison. Pie charts overload the working memory, forcing the person to make complicated calculations, and at the same time make a decision based on those comparisons.
What's the point of showing a pie chart when you want to compare data, except to say, "well, the slices are almost the same, but I'm not really sure which one is bigger, or by how much, or what order they are from largest to smallest. But the colors sure are pretty. Plus, I like round things. Oh,was I suppose to make some important business decision? Sorry."
Bad Choices and Bad Decisions
Interaction Designers, Graphic Artists, Report Builders, Software Developers, and Executives have all made the decision to use pie charts in their reports, software applications, and dashboards. It was a bad decision. It was a poor choice. There are always better options and choices, yet the designer still made the decision to use a pie chart. I'll expore why people make such poor choices in my upcoming blog entires. (Hint: It has more to do with emotions than with analytical thinking.)
I've outlined my opinions and arguments about the evils of using pie charts in "Countdown of Top 10 Reasons to Never Ever Use a Pie Chart." Each of my next 10 blog entries will support these arguments with illustrations, examples, and references to studies.
But my goal is not to continuously and endlessly rage against the evils of using pie charts. This blog is not about pie charts. This blog is about understanding why designers choose to use a pie chart. Why, when give better alternatives, and acknowledging the shortcomings of pie charts, do designers over and over again still freely choose to place a pie chart in a report?
As an extra treat and parting shot, check out the nice pie chart that Wikipedia uses to illustrate the United States population by state.
Remember, somebody chose to use this pie chart, with all its glorious colors, and post it on Wikipedia for all the world to see.
My next blog will give you a better alternative for displaying comparable data - the sorted bar chart.
