Until now, at TEFIPRO, we regularly used pie charts to display certain types of data. However, we have discovered the large number of errors and misunderstandings that this type of visualization generates.
Therefore, we are going to write a first article with the aim of “Emptying the Cup,” and in a subsequent one, we will explain how to display the same information more effectively.
As we mentioned, until now at TEFIPRO, we regularly used this type of chart. They seemed useful to us for indicating how different parts form a whole. However, following Stephen Few’s article – “Save the Pies for Dessert” [i], we have understood that it is not the best way to display information:
Let’s start by saying that pie charts are only easy to judge when the slices are close to 0%, 25%, 50%, 75%, and 100% (0°, 90°, 180°, 270°, 360°). For example:
It’s easy to recognize that the sales of “C” are 25%. However, in the following visualization, it is not as easy:
No values have changed, yet our ability to recognize the 25% is not the same.
Humans and their sense of sight are not well developed to accurately calculate angles or circular areas.
Naomi Robbins [ii] states:
“We judge angles when interpreting a pie chart, but we do not judge angles well. These judgments are biased; we underestimate acute angles and overestimate obtuse ones. Angles with the bisector in the horizontal plane appear larger than when the bisector is vertical.”
From here, we can use the different possibilities of the software to complete that information which our eyes are not able to capture, for example, by including the %.
And even the category name:
But by doing so, we are merely using a visually appealing type of chart and compensating for its deficiencies with additional information.
A chart is useful when it makes the relationship between data visible.
Another example of the human eye’s lack of skill with circular figures can be seen in the following visualization.
What is the relationship between the area of the small circle (2001) and the area of the large circle (2002)?
Most people give values between 6 and 50 (if you want to know the correct answer, it is at the end of the article).
This difficulty in discerning 2D circular areas is amplified in charts like the following:
Once we have confirmed that the human eye is not particularly skilled at calculating angles and circular areas, we encounter 3D pie charts:
The source data is the same; the orange slice is 40% while the gray slice is 25%. However, this perspective makes them appear equal. So if we are shown charts like these, we might suspect the intentions of the other party or their technical competence.
If our goal is to convey useful information aimed at improvement, pie charts are probably not the way to go. In the next installment, we will suggest a better approach.
(The relationship between the circular areas in the bubble chart is 16.)
[i] Save the Pies for Dessert
[ii] Naomi Robbins, Creating More Effective Graphs, Wiley, 2005, p. 49