Selection of subjects for study
This chapter deals with how subjects are selected for inclusion in analytical and intervention studies. To interpret a study, one must assess the problems that may have been overcome, or introduced, by the methods used to select subjects for the study. In observational cohort studies, one has to select subjects who are or have been exposed to the putative causal factor and a suitable comparison group who are unexposed or less exposed. In the case-control design, one has to select subjects in whom the outcome has occurred (cases) to compare with subjects in whom the outcome has not occurred (controls). In intervention studies, one selects subjects who are suitable and willing to have either the intervention being assessed or the alternative with which it is compared, which may be no intervention. Part 1 of this chapter covers the general principles, and Part 2 illustrates the application of these principles to each type of study. Self-test questions are provided at the end of the chapter.
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