In research about diseases or treatments, ‘Observational study’ refers to a study in which nature is allowed to take its course.
Observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator.
This is in contrast with controlled experiments, such as randomized controlled trials, where each subject is randomly assigned to a treated group or a control group before the start of the treatment.
There is a greater risk of selection bias than in experimental studies.
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