The thing that makes prospective cohort studies prospective is that they were designed prospectively, and subjects were enrolled and had baseline data collected before any of them developed any of the outcomes of interest. It goes without saying that analysis is always done retrospectively, because a span of time has to have elapsed before you can compare incidence. Of course, data analysis cannot take place until enough 'events' or 'outcomes' have occurred, so time must elapse, and the analyses will look at events that have occurred during the period of time from the beginning of the study until the time of the analysis or the end of the study. They design the questions and data collection procedures very carefully in order to have accurate information about exposures before disease develops in any of the subjects. Information is collected from all subjects in the same way using exactly the same questions and data collection methods for all subjects. The investigators then determine the initial or "baseline" characteristics, behaviors, and other "exposures" of all subjects at the beginning of the study. Eligible subjects had to meet certain criteria (inclusion criteria) to be included in the study as subjects. The investigators identified a cohort (group) of possible subjects who would be feasible to follow for a prolonged period. In each of these studies, the investigators wanted to study risk factors for common chronic diseases. The Framingham Heart Study, the Nurses Health Study, and the Black Women's Health Study are good examples of large, productive prospective cohort studies. Typically, the investigators have a primary focus, for example, to learn more about cardiovascular disease or cancer, but the data collected from the cohort over time can be used to answer many questions and test many possible determinants, even factors that they hadn't considered when the study was originally conceived. Combinations of these methods can also be used. The follow up can be conducted by mail questionnaires, by phone interviews, via the Internet, or in person with interviews, physical examinations, and laboratory or imaging tests. The subjects are then followed into the future in order to record the development of any of the outcomes of interest. In prospective cohort studies the investigators conceive and design the study, recruit subjects, and collect baseline exposure data on all subjects, before any of the subjects have developed any of the outcomes of interest. There are two fundamental types of cohort studies based on when and how the subjects are enrolled into the study: Prospective Cohort Studies: Prospective Versus Retrospective Cohort Studies
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