![]() ![]() The analysis always occurs after a certain number of events have taken place. Since the data analysis occurs after some outcomes have occurred, some students mistakenly would call this a retrospective study, but this is incorrect. Pitfall: Note that in these prospective cohort studies a comparison of incidence between the groups can only take place after enough time has elapsed so that some subjects developed the outcomes of interest. Key Concept: The distinguishing feature of a prospective cohort study is that at the time that the investigators begin enrolling subjects and collecting baseline exposure information, none of the subjects has developed any of the outcomes of interest. Alternatively, one could group subjects based on their body mass index (BMI) and compare their risk of developing heart disease or cancer. For example, one could identify smokers and non-smokers at baseline and compare their subsequent incidence of developing heart disease. In this way, investigators can eventually use the data to answer many questions about the associations between "risk factors" and disease outcomes. over a period of time, usually for years, to determine if and when they become diseased and whether their exposure status changes. ![]() After baseline information is collected, subjects in a prospective cohort study are then followed "longitudinally," i.e. The investigators design the questions and data collection procedures carefully in order to obtain accurate information about exposures before disease develops in any of the subjects. In a prospective study like the Nurses Health Study baseline information is collected from all subjects in the same way using exactly the same questions and data collection methods for all subjects. ![]()
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