Discussion:
Discuss the various options for sampling and cohort formation within epidemiologic cohort studies. As a practicing epidemiologist, how would you go about selecting the best sampling and cohort formation options for a particular study?
CLASSMATE'S POST
Prospective cohort (concurrent; longitudinal study) - An investigator identifies the study population at the beginning of the study and accompanies the subjects through time. In a prospective study, the investigator begins the study at the same time as the first determination of exposure status of the cohort. When proposing a prospective cohort study, the investigator first identifies the characteristics of the group of people he/she wishes to study. The investigator then determines the present case status of individuals, selecting only non-cases to follow forward in time. Exposure status is determined at the beginning of the study.
• Problems: loss to follow up; differential nonresponse; loss of funding support; continually improving methods for detecting exposure (leading to greater misclassification than would be expected in current practice)
• Examples: Framingham Study; Nurses Health Study; National Health and Nutrition Examination Study Follow up Study. These are all studies where case status was determined at the beginning of the cohort and cases eliminated from the study. Exposure was then measured who were followed over a period of time until reaching the study endpoint. A member of the cohort reaches the endpoint either by dying, becoming a case, or reaching the end of the study period. A subject can also be lost to follow-up over the course of the study. The investigator progresses through time with the subjects in a prospective cohort study. Such a study may also be called a longitudinal or a concurrent study, as opposed to a retrospective cohort study.