Cohort studies are advantageous because they can be used to measure inci-
dence and to study many outcomes. Generally, cohort studies are quite large,
so they are a good design to study rare exposures and can readily establish
that an exposure preceded the disease. Cohort studies are less vulnerable to
recall bias.
Cohort studies have disadvantages as well. They tend to be expensive be-
cause of the need for a large sample size and their longitudinal nature. They
require a significant amount of time depending on the disease of interest and
are impractical for rare outcomes. Because cohort studies are longitudinal,
the threat of mortality is significant because subjects drop out, move, or
even die during the course of the study. Exposures during the course of a
cohort study may change. For example, a person may begin a study smoking
two packs per day but during the study may cut back to half a pack per day
or quit smoking entirely. These changes can be controlled if the investiga-
tor measures exposures more frequently throughout the study rather than
simply at baseline.Intervention Studies
A third design commonly used in epidemiology is an intervention study. The
key feature of this design is that the investigator manipulates the exposure of
interest and then assigns subjects to one or more exposure groups, including
a placebo or a group receiving the standard of care. Like experimental de-
signs, an intervention study has the hallmark feature of random assignment
of individuals into treatment groups. Randomization is used to control for the
effects of confounding variables. Like cohort studies, the statistical test used
for intervention studies is the RR.
There are advantages and disadvantages of intervention studies. The advantages
include control over the exposure, randomization to control for potential con-
founding variables, and blinding to reduce bias. Some disadvantages associated
with intervention studies are costs and time. The study subjects may experience
the Hawthorne effect. Because intervention studies are often time-consuming,
subjects may become noncompliant or withdraw from the study, which biases
outcomes. For ethical reasons, not all exposures are able to be manipulated,
and therefore this design would not be appropriate.KEY TERM
intervention study:
In epidemiology,
a study that has a
treatment that can
be manipulated by
the researcherWhen reading research reports, have you encountered any reports of incidence or prevalence?
How about OR or RR?CRITICAL THINKING EXERCISE 8-2
212 CHAPTER 8 Epidemiologic Designs: Using Data to Understand Populations