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Perhaps except
clinical trials, other study designs used by epidemiologists are generally less
experimental in nature. These include descriptive studies, case-control studies, cohort
studies, cross-sectional studies, and ecological studies. Descriptive studies make use of
available data to examine how mortality, morbidity, and other exposure- or risk-related
rates vary according to certain demographic, geographic, and environmental variables. The
other study designs listed here basically are for analytical epidemiology, in which the
associations between exposures (including all relevant risk factors such as age and sex)
and outcomes are tested or measured. Cohort studies involve the follow up of a group of
healthy people (cohort) for a certain time period to ascertain the occurrence of
health-related events in relation to the exposure or risk factor of interest. Case-control
studies, on the other hand, compare the odds of usually past exposure to a suspected risk
factor between diseased individuals (cases) and nondiseased individuals (controls). In a
cross-sectional study, a reference population is examined at a given point in time to
measure the prevalence of disease and the level of exposure of interest. Ecological
studies are conducted to determine the number of exposed persons and the number of cases
that may or may not be related, usually due to the lack of the ability to determine the
number of exposed cases. Thus, in ecological studies, the unit of analysis is the
group, not individuals. |
