Page 108 - Statistics for Dummies

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Part II: Number-Crunching Basics
Take Another Little Piece of My Pie Chart
A pie chart takes categorical data and breaks them down by group, showing
the percentage of individuals that fall into each group. Because a pie chart
takes on the shape of a circle, the “slices” that represent each group can
easily be compared and contrasted.
Because each individual in the study falls into one and only one category, the

sum of all the slices of the pie should be 100% or close to it (subject to a bit of
rounding off). However, just in case, keep your eyes open for pie charts whose
percentages just don’t add up.
Tallying personal expenses
When you spend your money, what do you spend it on? What are your
top three expenses? According to the U.S. Bureau of Labor Statistics 2008
Consumer Expenditure Survey, the top six sources of consumer expendi-
tures in the U.S. were housing (33.9%), transportation (17.0%), food (12.8%),
personal insurance and pensions (11.1%), healthcare (5.9%), and entertain-
ment (5.6%). These six categories make up over 85% of average consumer
expenses. (Although the exact percentages change from year to year, the list
of the top six items remains the same.)
Figure 6-1 summarizes the 2008 U.S. expenditures in a pie chart. Notice that
the “Other” category is a bit large in this chart (13.7%). However, with so
many other possible expenditures out there (including this book), each one
would only get a tiny slice of the pie for itself, and the resulting pie chart
would be a mess. In this case, it is too difficult to break “Other” down further.
(But in many other cases you can.)
Ideally, a pie chart shouldn’t have too many slices because a large number
of slices distracts the reader from the main point(s) the pie chart is trying to
relay. However, lumping the remaining categories into one slice that’s one of
the largest in the whole pie chart leaves readers wondering what’s included in
that particular slice. With charts and graphs, doing it right is a delicate balance.
Bringing in a lotto revenue
State lotteries bring in a great deal of revenue, and they also return a large
portion of the money received, with some of the revenues going to prizes and
some being allocated to state programs such as education. Where does lot-
tery revenue come from? Figure 6-2 is a pie chart showing the types of games
and their percentage of revenue as recently reported by Ohio’s state lottery.
(Note the slices don’t sum to 100% exactly due to slight rounding error.)

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