Page 320 - Statistics for Dummies

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Part V: Statistical Studies and the Hunt for a Meaningful Relationship
Graphing joint distributions
To graph a joint distribution from a two-way table, you make a single pie
chart with four slices, representing each proportion of the data that falls
within a row-column combination. Groups containing more individuals get a
bigger piece of the overall pie, and hence get more weight when all the votes
are counted up. Figure 19-2 is a pie chart showing the joint distribution for
the pet camping survey data.
All Campers
15.0%
20.0%
Pet campers, support pet section
Pet campers, oppose pet section
Non-pet campers, support pet section
Non-pet campers, oppose pet section
10.0%
Figure 19-2:
Pie chart
showing the
joint distri-
bution of the
pet camping
55.0%
and opinion
variables.

From the pie chart shown in Figure 19-2, you see some results that stand out.
The majority of campers in this sample (0.55 or 55%) don’t camp with pets
and support a separate section for pets. The smallest slice of the pie repre-
sents those campers who camp with pets and are opposed to a separate sec-
tion for pets (0.10 or 10%).
A joint distribution gives you a breakdown of the entire group by both vari-
ables at once and allows you to compare the cells to each other and to the
whole group. The results in Figure 19-2 show that if they were asked to vote
today as to whether or not to have a pet section, when all the votes were
added up, most of the weight would be placed on the opinions of non–pet
campers, because they make up the majority of campers in the survey (70%,
according to Table 19-4), and the pet campers would have less of a voice,
because they are a smaller group (30%).

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