Читать книгу The Pocket Guide to Critical Appraisal - Iain K. Crombie - Страница 16

The defining characteristic of cohort studies is the element of time: in cohort studies time flows forwards. A set of individuals is identified at one point in time and followed up to a later time to ascertain what has happened. These studies are called prospective cohort studies. When studying the impact of some exposure, cohort studies often have a comparison or control group. The controls are usually identified at around the same time and followed for the same length of time. This type of cohort study is called a concurrent cohort study.

Many cohort studies do not have a control group; instead they can make internal comparisons. For example, a cohort could recruit a large sample from the general population and identify who smoked and who did not. The whole sample would be followed up for several years to determine whether lung cancer occurs more frequently in smokers than in non‐smokers.