7.2.1.1 Cohort Studies

Cohort studies are the “gold standard” of observational study designs and prospective cohort studies appear the highest on evidence hierarchies of observational study designs (Thiese 2014). These longitudinal studies are typically used to analyse relationships between exposures and disease by comparing the outcomes between two groups over time, where individuals in one group are exposed to a common event or characteristic, such as a risk factor, and the other group are not. Sampling in cohort studies is based on the presence or absence of an exposure or characteristic, and participants are followed over time to observe development of any disease or health outcomes. A prospective cohort study begins with the exposure of interest, and participants are followed forward through time to observe any outcomes that may occur. Conversely, a retrospective cohort study generally begins after the outcomes of interest have already been recorded; a researcher may sift through patient records or data that is already available and groups patients according to exposures, and identifies any differences in outcomes. Cohort studies enable observations of a large number of people over a long period of time.