Page 113 - Statistics for Dummies

Chapter 6: Getting the Picture: Graphing Categorical Data
Evaluating a pie chart
The following tips help you taste test a pie chart
✓ Look for a reported total number of units
for statistical correctness:
(people, dollar amounts, and so on) so that
you can determine (in essence) how “big”
✓ Check to be sure the percentages add up to
the pie was before being divided up into the
100% or very close to it (any round-off error
slices that you’re looking at.
should be very small).
✓ Avoid three-dimensional pie charts; they
✓ Beware of slices of the pie called “Other”
don’t show the slices in their proper pro-
that are larger than many of the other
portions. The slices in front look larger than
slices.
they should.
Raising the Bar on Bar Graphs 97
A bar graph (or bar chart) is perhaps the most common data display used
by the media. Like a pie chart, a bar graph breaks categorical data down by
group. Unlike a pie chart, it represents these amounts by using bars of differ-
ent lengths; whereas a pie chart most often reports the amount in each group
as percentages, a bar graph uses either the number of individuals in each
group (also called the frequency) or the percentage in each group (called the
relative frequency).
Tracking transportation expenses
How much of their income do people in the United States spend on transporta-
tion to get back and forth to work? It depends on how much money they make.
The Bureau of Transportation Statistics (did you know such a department
existed?) conducted a study on transportation in the U.S. recently, and many
of its findings are presented as bar graphs like the one shown in Figure 6-5.
This particular bar graph shows how much money is spent on transporta-
tion for people in different household-income groups. It appears that as
household income increases, the total expenditures on transportation also
increase. This makes sense, because the more money people have, the more
they have available to spend.

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