The other day, I published, Effectiveness of Chart Design: See for Yourself and the next day, The Big Apple Pie Chart. There were several comments in support of some limited cases where pie charts are effective.

Edward Tufte

In response to an interesting discussion, DivinoAG says:

I don’t think anyone ever checks the watch to know what time it is right now (unless you need to fill that on a form, there’s hardly any use for that information), but to see how much time they have until something else happens. Because of that, using a digital watch is a pain for me because I need to mentally calculate the difference between the current time and the time I actually am interested, and that only gives me another number which I find harder to quantify than having an image.

But with a analog watch I can look at the current hand position, the number for whatever time I’m waiting for, and create a mental image that is nothing but a pie chart, which I can then, almost instantly, understand as “a lot” or “a little” time.

That’s how pie charts are useful: they provide very clear contrast between few entities (two in this case, “time left” and “other time”), and allow the viewer to gather instant information when granular detail is not needed. It’s usually not important if I have 10 or 11 minutes to lunch time, only if it’s 10 or 30 minutes.

This seems consistent with the very narrow class of situations where a pie chart might be effective. It certainly does not explain the widespread usage of the pie chart.

What has the literature said?

Pie charts are not without their strengths. The primary strength of a pie chart is the fact that the message “part-to-whole relationship” is built right into it in an obvious way. Children learn fractions by looking at pies sliced in various ways and decoding the ratio (quarter, half, three quarters, etc.) of each slice.

Stephen Few, Save the Pies for Dessert, Perceptual Edge, August 2007, page 1.

Despite the obvious nature of a pie charts (sic) message, bar graphs provide a much better means to compare the magnitudes of each part. Pie charts only make it easy to judge the magnitude of a slice when it is close to 0%, 25%, 50%, 75%, or 100%. Any percentages other than these are difficult to discern in a pie chart, but can be accurately discerned in a bar graph, thanks to the quantitative scale.

Stephen Few, Save the Pies for Dessert, Perceptual Edge, August 2007, page 2.

Part-to-whole relationships are commonly displayed as pie charts. This is unfortunate. Pie charts force us to compare either the 2-D areas formed by each slice or the angles formed by each where the slices meet in the center. Visual perception handles neither of these comparisons easily or accurately. . . . You might be tempted to object, “This could be solved by displaying the values as text next to each slice” . . . This is true, but what’s the point of using a graph — a visual representation of the quantitative data — if we must rely on the printed values to make sense of it?

Stephen Few, Now You See It, 2009, pp 191-192.

We have seen . . . (ed. earlier) . . . that a pie chart does not communicate very effectively. We noted that dot plots and other charts that require judgments of position along a common scale are better for getting quantitative information across reliably. But we also noted that there may be other reasons to draw graphs, such as to make a page more inviting . . . Many financial reports use pie charts with there or four wedges to decorate the page . . . Adding a data label to a pie chart will make the information clearer. But realize, of course, that you are communicating through the text in the data labels rather than through the chart itself.

Naomi Robbins, Creating More Effective Graphs, 2005, pp 350-351.

You might be wondering, aside from the natural way that pie charts suggest a part-to-whole relationship, do they have anything else going for them? I have read every research study that I could find that tested the effectiveness of pie charts versus other means of displaying quantitative data, beginning with one that was done in 1926 by W. C. Ells, and have found only one advantage that can confidently be attributed to pie charts. Unfortunately, this one strength is rarely if ever useful. . . . . The one slight advantage of a pie chart over a bar graph involved a task that required subjects to answer the question, “Which is greater, A+B or C+D?”, while examining one of the three charts below

Stephen Few, Save the Pies for Dessert, Perceptual Edge, August 2007, page 13.

and finally, probably the most frequently cited:

One of the prevailing orthodoxies of this forum – one to which I whole-heartedly subscribe – is that pie charts are bad and that the only thing worse than one pie chart is lots of them.

Edward Tufte

Conclusion

In the end, I conclude that outside of decorative purposes, pie charts have little effective data communication, and almost always would be better replaced with text, a table, or an alternate graph. I believe pie charts are so prevalent because people “like them” and are in the habit of using them (although I have no data to back this up). It is certainly possible to like things that are neither effective nor efficient. I try to use them as infrequently as possible.

Comments