112 PART 3 Getting Down and Dirty with Data
Summarizing and Graphing
Categorical Data
A categorical variable is summarized by tallying the number of participants in
each category and expressing this number as a count. You might also compute a
percentage of the total number of participants in all categories combined. So a
sample of 422 participants can be summarized by health insurance type, as shown
in Table 9-1.
The joint distribution of participants between two categorical variables is sum-
marized by a cross-tabulation (or cross-tab). Table 9-2 shows an example of a
cross-tab of the same participants in our example with type of health insurance
on one axis, and urban-rural classification of their residence on the other.
TABLE 9-1
Study Participants Categorized by Health Insurance Type
Health Insurance Type
Count
Percent of Total
Commercial
128
30.3%
Public
141
33.4%
Military
70
16.6%
Other
83
19.7%
Total
422
100%
TABLE 9-2
Cross-Tabulation of Participants by Two
Categorical Variables
Health Insurance Type
Commercial
Public
Military
Other
Total
Urban-Rural
Classification of
Residence
Rural
60
60
34
42
196
Urban
68
81
36
41
226
Total
128
141
70
83
422