This chapter discusses two large classes of observational study—prospective and retrospective. Each class comes with inherent variations in methodology that carry the potential for sufficient bias to threaten the validity of their results. The prospective observational study is most similar to a randomized trial, but the exposure occurs during the course of everyday life instead of by randomized allocation. In prospective designs, investigators follow the exposed study subjects over time to note how many develop the disease of interest. The disease rates are then compared to a group of unexposed people. Retrospective studies, on the other hand, are backward-looking, in that a group of people with the disease are examined to ascertain whether they were exposed to the agent under study. Their exposure rate is compared either to standard population rates or to another group of similar subjects who are asked about their past exposures.
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