Page 322 - Statistics for Dummies

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Part V: Statistical Studies and the Hunt for a Meaningful Relationship
Table 19-7
Conditional Distributions of Opinion for

Pet Campers versus Non–Pet Campers
Support Pet
Oppose Pet
Total
Section Policy
Section Policy
Pet Campers
10 ÷ 30 = 0.33
20 ÷ 30 = 0.67
1.00
55 ÷ 70 = 0.79
1.00
Non–Pet Campers
15 ÷ 70 = 0.21
Notice that Table 19-7 differs from Table 19-6 in the earlier section “Calculating
joint distributions” in terms of how the values in the table add up. This rep-
resents the key difference between a joint distribution and a conditional
distribution that allows you to make fair comparisons using the conditional
distribution:
✓ In Table 19-6, the proportions in the cells of the entire table sum to 1
because the entire group is broken down by both variables at once in a
joint distribution.
✓ In Table 19-7, the proportions in each row of the table sum to 1 because
each group is treated separately in a conditional distribution.
Graphing conditional distributions
One effective way to graph conditional distributions is to make a pie chart for
each group (for example, one for pet campers and one for non–pet campers)
where each pie chart shows the results of the variable being studied (opinion:
yes or no).
Another method is to use a stacked bar graph. A stacked bar graph is a spe-
cial bar graph where each bar has a height of 1 and represents an entire
group (one bar for pet campers and one bar for non–pet campers). Each bar
shows how that group breaks down regarding the other variable being stud-
ied (opinion: yes or no).
Figure 19-3 is a stacked bar graph showing two conditional distributions. The
first bar is the conditional distribution of opinion for the pet camping group
(row 1 of Table 19-7) and the second bar represents the conditional distribu-
tion of opinion for the non–pet camping group (row 2 of Table 19-7).
Using Table 19-7 and Figure 19-3, first look at the opinions of each group.
More than 50% of the pet campers support the pet section (the exact number
rounds to 67%), so you say the majority of pet campers support a pet sec-
tion. Similarly, the majority of non–pet campers (about 79%, way more than
half) support a pet section.

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