A cohort study is an observational study in which researchers follow a predetermined group of people called a cohort over time to record outcomes such as development of disease or effects from certain exposures. The cohort is often divided into different groups based on demographics, clinical characteristics, or exposures, such as behaviors, environmental factors, genetics or medical treatments.
There are two main types of cohort studies. Retrospective cohort studies use existing data to identify the cohort that has been exposed to past exposures, and then researchers track their outcomes. The exposures and outcomes have already occurred when the data are collected, so the researchers are looking back in time. Prospective cohort studies follow participants forward in time. They are enrolled at the start of the study, and both exposures and outcomes are tracked from that point onward.
Cohort studies cannot show causation and are subject to the same limitations as other observational studies. They also may not have a control group, which can further limit how much information can be learned from them or how generalizable the conclusions are.