Pie Charts Have No Place in Market Research
Posted by Vovici Blog on Wed, Mar 30, 2011
The goal of conducting a survey or other market-research study is to come up with useful information and communicate it effectively. Pie charts miscommunicate data and therefore should be avoided.
From the following chart, can you list the products by order of preference for each question?
You can’t, and it’s not your fault. Our visual perception does not always accurately reflect what we are seeing: we have difficulty comparing and contrasting visual area, we struggle when comparing angles, and with pies we are forced to compare wedges that aren’t adjacent to one another:
- Stanley Smith Stevens studied the limits of human perceptions across many different dimensions. He found that we accurately perceived differences in brightness, length and cold, but we overestimate small differences in visual area and progressively underestimate large differences in area. (See Stevens’ power law for more details.)
- William Cleveland, in The Elements of Graphing Data, relates the results of an experiment where 51 subjects judged 40 pairs of values on bar charts and then on pie charts. In almost all cases, the data was interpreted more accurately using bar charts (the exceptions were pie charts that compared one slice to the whole when that slice was 25%, 50% or 75%).
- As Edward Tufte writes in The Visual Display of Quantitative Information, “A table is nearly always better than a dumb pie chart; the only thing worse than a pie chart is several of them, for then the viewer is asked to compare quantities located in spatial disarray both within and between pies. Given their low data-density and failure to order numbers along a visual dimension, pie charts should never be used.”
Here’s a comparison of the same data shown with pie charts and column charts:
The bar charts ("column charts" in many graphing packages) make the small differences in products easily recognizable. Pie charts needlessly confuse.
For far more entertaining takedowns of the pie chart, see Annie Pettit’s “Pie Charts – Our Evil Friend” and Coda Hale’s Google Analytics rant, wherein he says, “Pie charts are the information visualization equivalent of a roofing hammer to the frontal lobe. They have no place in the world of grownups, and occupy the same semiotic space as short pants, a runny nose, and chocolate smeared on one’s face.”
Chocolate, or pie, perhaps.