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The two main
classes of study design are experimental and non-experimental.
In an experimental study, conditions (usually the treatments)
are under the direct control of the investigator. These studies
test the effectiveness of a treatment or intervention by comparing
the outcome (for example the frequency and intensity of a symptom)
in the experimental group with the outcome in the control group.
In a randomized controlled trial, individuals are randomly allocated
to the groups.
Non-experimental
studies observe something that naturally occurs. Descriptive studies
describe patterns of disease, symptoms, or problems in a population.
Analytic studies examine an association between a problem of interest
and other variables, and possible causative factors are examined.
These last groups include cross-sectional, longitudinal, and case-control
designs.
Symptoms can be studied in both experimental and non-experimental
studies. The principles of detecting, measuring, and recording
symptoms are shared in all these designs. The remainder of this
chapter focuses on two common non-experimental designs used in
symptom studies: cross-sectional and longitudinal.
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