Pie Chart Research

This post is an aside to my main post on pie charts.

In my quest to understand this controversy I ended up reading a number of research papers on the topic. Here in chronological order are a few notes from relevant ones:

1984: I covered Cleveland and McGill’s seminal work on graphical perception¹ in my main article on pie charts. In the paper, they investigated a number of representations for accuracy in various tasks. Overall they found pie charts and stacked bar charts to be roughly equivalent in performance, with neither being particularly accurate. Instead they recommended the use of a bar chart or dot chart to encode the same information.

1991: Spence and Lewandowsky² found bar charts and pies to be mostly equivalent. Bar charts were better at estimating direct magnitude and pies were better for comparisons of combinations of proportions (is segment A bigger than segments B + C?).

1998: Hollands and Spence³ looked at performance judging proportions of a whole and found pie charts the most accurate and equally speedy as stacked bars. Bar charts were least accurate and their speed decreased with the number of bars. They suggest that the poor bar chart performance is due to the users needing to mentally sum the bar lengths to determine the overall length, so perhaps adding an explicit 100% marker to the chart might avoid this issue.

2006: There’s a lot of interesting research looking at the use of visualizations to convey medical health risk information. Ancker et al⁴ compared stacked bars, pair of numbers, icon arrangements and pie charts. They found pie charts and random icons to be the slowest and least accurate when judging the larger of two quantities. Pie charts were superior when mental summation of slices was required.

2008: Schonlau et al⁵ compared bar charts, pie charts and tables, asking participants to identify largest/smallest categories and estimate the perecentage of the largest. They found that tables were the most accurate and bars pies roughly equally less accurate. They also found that adding a gratuitous 3rd dimension negatively affected performance for pie charts but not for bar charts.

2010: Heer and Bostock⁶ retested Cleveland and McGill’s findings asking participants to identify the smaller of two values and estimate the percentage the smaller was of the larger. Again they found that bar charts (position) performed better than both pies (angle) and stacked bars (length), which all performed better than sized circles or rectangles (area).

2012: Schonlau revisited⁷ the issue with Peters in 2012. Graphs were found better for estimating differences and tables better for equality and sums. Stacked bar charts are to be avoided (“Comprehension… was worse for all the but the easiest tasks”). Pie charts got the backhanded compliment of “pie charts never assisted comprehension, they impaired comprehension on fewer than half of the tasks.” They were as good as tables for judging which of two categories was larger and worse for estimating percentage or difference of percentage. Of note, their paper includes a concise summary of some previous research (some mentioned here).

2016: Recently Skau and Kosara looked at variations of pie charts to see how design choices affect their performance. They found both pie and doughnut charts to perform fine and listed a few design guidelines (e.g. avoid exploded pie charts if segment summation is required; keep the radius consistent between segments).