Evidence-Based Practice for Nurses

a historical exposure and then follow the sample forward to the present time
to determine whether the disease is present.

In a cohort study, the investigator compares the probability of disease in
individuals who are exposed to the probability of disease in individuals who
are not exposed. The statistical test used for a cohort study is relative risk (RR).
For example, consider the Mississippi Department of Health study that was
previously described. Investigators were interested in examining the relationship
between heart disease and smoking. In the previous example of a case-control
study, the investigators sought individuals who had AMIs. In contrast, when
using a cohort design, investigators select individuals based on whether or
not they smoke. Therefore, medical records at Jackson County Hospital were
examined and 1,000 patients were invited to join the cohort study. Of the 1,000
patients, investigators found 550 patients who smoked, and 450 patients who
did not have a history of smoking but who were similar to the smokers in all
other demographic areas. They followed these individuals to see who did and
did not have an AMI. Of the 550 patients who smoked, 400 patients developed
an AMI compared to only 100 patients who never smoked developing an AMI.
These data are recorded in the 2 × 2 table as follows:

AMI No AMI Total

Smoking

(Cases)

400 150 550

No Smoking

(Controls)

100 350 450

Total 500 500 1,000

An RR was calculated using the following formula:

RR

400

550

100

450

0.727

0.222

====3.27

a

a+b

c

c+d

The interpretation of an RR is the same as an OR. An RR equal to 1.0 indi-
cates the probability of disease among the exposed and the nonexposed is
identical; therefore, there is no association. An RR greater than 1.0 indicates
there is a greater probability of disease among the exposed; therefore, there
is an association. An RR less than 1.0 indicates there is decreased probability
of disease among the exposed or a protective effect. For this example, the
RR value indicates that smokers were 3.27 times more likely to have an AMI
compared to nonsmokers.

KEY TERM

relative risk: The

statistic reported

by epidemiologists

when they conduct

a cohort study

8.6 Analytic Study Designs 211