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 Define " loss to follow-up" and explain what effects it may have on a study.Explain what the " healthy worker effect" is.Explain the differences among the following types of comparison groups:.Explain what is meant by the term "comparison group".Differentiate between a specific/special exposure cohort and a general cohort.Explain the factors that should be considered in selecting subjects for a cohort study.Explain the advantages and disadvantages of the cohort design in general and the strengths and weaknesses of retrospective and prospective cohort studies.Distinguish between "closed" and "open" cohorts.Define what a cohort study is and explain its key features.Upon successful completion of this section of the course, the student will be able to: It must be established that a cohort did not have the outcome of interest at the beginning of the observation period, and the cohort needs to be examined again to determine whether or not the outcome subsequently developed, i.e., the incidence in each of the exposure groups. (We can refer to the groups being compared as exposure cohorts.) Cohorts may be identified retrospectively or prospectively, but in either case the outcome status needs to be established at least twice. The characteristic feature of a cohort study is that the investigator identifies subjects at a point in time when they do not have the outcome of interest and compares the incidence of the outcome of interest among groups of exposed and unexposed (or less exposed) subjects.