is identical; therefore, there is no association between the exposure and the
disease. When an OR is greater than 1.0, there is a greater probability of disease
among the exposed; therefore, there is an association between the exposure and
the disease. An OR of less than 1.0 indicates there is decreased probability of
disease among the exposed or a protective effect. In this example, the value of
9.33 shows that there is an association between smoking and having an AMI.
Individuals who smoke were 9.33 times more likely to have an AMI compared
to those who did not smoke.
Case-control studies can be advantageous. They are expedient, require a
small sample size, and are relatively inexpensive. Case-control studies are used
to examine rare diseases and situations that involve individuals who have had
many exposures. Another advantage is that the risk of attrition is low because
participants are asked to recall exposures and thus do not leave the study.
Case-control studies have disadvantages as well. They cannot measure in-
cidence. These designs are able to examine only one disease rather than many.
The design is not conducive to measuring rare exposures. Because of the ret-
rospective design, recall bias is a threat. This bias can be especially significant
when cases are searching for a reason to explain why they acquired the disease.
They may be more likely to report potential causes and risk factors as well as
exposures that may not have happened to them.Cohort Studies
Cohort studies are natural experiments. One famous cohort study is the
Nurses’ Health Study that began in 1976. The primary goal of this study was
to examine diet and lifestyle risk factors and their associations with cancer,
heart disease, and other chronic diseases. The study was initiated in 1976 with
120,000 female nurses between the ages of 30 and 55 years. The population
of the study was ethnically and racially diverse, although all had a higher
education than the general population of women. Now in its third version,
nurses continue to be followed and outcomes have been added (Nurses’
Health Study, n.d.).
Unlike case-control studies where individuals are selected based on the pres-
ence or absence of disease, in cohort studies individuals are selected based on
their exposure. The investigator selects a sample group that is representative
of the target population. Once the sample has been selected, the investigator
assigns individuals to groups based on their exposure status. Exposure status
is hypothesized to be positively or negatively associated with risk of disease.
Investigators can use either prospective or retrospective designs. Prospective
designs follow individuals from the time they enroll in the study until they de-
velop the disease. When investigators use retrospective designs, they determineKEY TERM
cohort studies:
Quasi-experimental
studies using two
or more groups;
epidemiologic
designs in which
subjects are
selected based on
their exposure to a
determinant210 CHAPTER 8 Epidemiologic Designs: Using Data to Understand Populations