of exposure and disease; therefore, the measure of association for cross-sectional
studies is the prevalence ratio (PR). The PR estimates the magnitude of the associa-
tion between the exposure and the disease. The PR does not inform the nurse that
the exposure causes the disease because these data are collected at the same time.
It is important to review how data are organized in epidemiology. Data are put
into a 2 × 2 table, also known as a contingency table. The 2 × 2 table categorizes
subjects in a study based on exposure and disease status, where A equals the
number of people with both the exposure and disease; B equals the number of
people with the exposure, but not the disease; C equals the number of people
with the disease, but not the exposure; and D equals the number of people with
neither the disease nor the exposure. It is very important always to place the
same information in the same cell; otherwise, the calculation will be incorrect.
Disease No Disease Total
Exposure A B A+B
No Exposure C D C+D
Total A+C B+D A+B+C+DFor example, the Texas Department of Health sent a questionnaire to 1,000
adults to inquire about their blood pressure and exercise patterns. Analysis
of the surveys showed that 100 cases of hypertension (HTN) were reported
among those who exercise more than three times per week, while 400 cases
of HTN were found among those who reported no exercise. Of the 500 cases
without HTN, 300 reported exercising more than three times per week and 200
reported no exercise. These data were entered into a 2 × 2 table as shown here.
HTN No HTN Total
Exercise 100 300 400
No Exercise 400 200 600
Total 500 500 1,000Using these data, a PR was calculated using the following formula:PR100
400
400
6000.25
0.66= + 0.38
+===a
ab
c
cdIn this study, the exposure of exercising more than three times per week
provided a protective effect for HTN. With protective effects, the PR is sub-
tracted from 1 and multiplied by 100 to derive the percentage of the reduced
8.5 Descriptive Study Designs 207