Field of medicine concerned with the determination of causes, incidence, and characteristic behavior of disease outbreaks affecting human populations. It includes the interrelationships of host, agent, and environment as related to the distribution and control of disease.
Some medical studies follow a cohort of many people for many years, gathering data on them and sometimes their relatives, so reducing the selection biases that creep into single-snapshot studies.
Please submit the websites of longitudinal studies. The websites of papers based on them usually belong with the specific disease being studied. However, papers advising on the structure of longitudinal studies do belong here.