Graphics - Common Errors in Statistics - page 118

Rule 7: Do not use pie charts unless the sum of the entries is sci-
entifically meaningful and of interest to the reader.
On the other hand, the pie chart is an effective display for illustrating
proportions. This is especially true when we want to focus on a particular
slice of the graphic that is near 25% or 50% of the data since we humans
are adept at judging these size portions. Including the actual value as a
text element decorating the associated pie slice effectively allows us to
communicate both the raw number along with the visual clue of the
proportion of the total that the category represents. A pie chart intended
to display information on all sections where some sections are very small is
very difficult to interpret. In these cases, a table or bar chart is to be
preferred.
Additional research has addressed whether the information should be
ordered before placement in the pie chart display. There are no general
rules to follow other than to repeat that humans are fairly good at identi-
fying pie shapes that are one-half or one-quarter of the total display. As
such, a good ordering of outcomes that included such values would strive
to place the leading edge of 25% and 50% pie slices along one of the
major north-south or east-west axes. Reordering the set of values may
lead to confusion if all other illustrations of the data used a different
ordering, so the graphic designer may ultimately feel compelled to repro-
duce other illustrations.
THREE RULES FOR EFFECTIVE DISPLAY OF
SUBGROUP INFORMATION
Graphical displays are very effective for communication of subgroup infor-
mation—for example, when we wish to compare changes in median family
income over time of African-Americans and Hispanics. With a moderate
number of subgroups, a graphical presentation can be much more effective
than a similar tabular display. Labels, stacked bar displays, or a tabular
arrangement of graphics can effectively display subgroup information.
Each of these approaches has its limits, as we will see in the following
sections.
In Figure 8.11, separate connected polylines easily separate the sub-
group information. Each line is further distinguished with a different plot-
ting symbol. Note how easy it is to confuse the information due to the
inverted legend. To avoid this type of confusion, ensure that the order of
entries (top to bottom) matches that of the graphic.
Rule 8: Put the legend items in the same order they appear in the
graphic whenever possible.