Background - Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics - page 38

2.7.4 Pie or Donut Charts
Pie or donut charts ( Figure 2.14 ) illustrate the parts or percentages of a whole.
They can be useful any time you want to illustrate the relative proportions of the
parts of a whole to each other (e.g., how many participants in a usability test suc-
ceeded, failed, or gave up on a task). Here are some key principles for their use.

• Pieordonutchartsareappropriateonlywhenthepartsaddupto100%.

You have to account for all the cases. In some situations, this might mean
creating an “other” category.

• Minimize the number of segments in the chart. Even though the bad

example in Figure 2.14 is technically correct, it's almost impossible to
make any sense out of it because it has so many segments. Try to use
no more than six segments. Logically combine segments, as in the good
example, to make the results clearer.

• Inalmostallcases,youshouldincludethepercentageandlabelforeach

segment. Normally these should be next to each segment, connected by
leader lines if necessary. Sometimes you have to move the labels manu-
ally to prevent them from overlapping.

% of Pages with Accessibility Errors

0 Errors
10%

>10 Errors
25%

1-2 Errors
12%

3-10 Errors
53%

% of Pages with Accessibility Errors

0 Errors
1 Error
2 Errors
3-4 Errors
5-6 Errors
7-8 Errors
9-10 Errors
11-15 Errors

Figure 2.14 Good (top) and bad (bottom) examples of pie or donut charts for the same data. Mistakes in the
bad version include too many segments, poor placement of the legend, not showing percentages for each
segment, and using 3D, for which the creator of this pie chart should be pummeled with a wet noodle.