Arms, Groups, and Cohorts

In a clinical study, participants are often divided into groups.

In interventional studies, participants are prospectively assigned to “arms” and receive different interventions. Assignment to an arm is typically random. Multiple arms allow the effects of different interventions to be compared. Participants in a control arm may receive no intervention, placebo, or an intervention with a known effect.

In observational studies, a pre-defined population may be observed over time and is termed, a cohort. A cohort consists of individuals with some common characteristic(s), for example, age, place of residence/employment, or medical condition. A cohort is observed over time to determine if the presence or absence of an exposure is associated with a health outcome or condition. Other observational studies may compare those with a condition (cases) to those without a condition (control) and evaluate differences in exposures, treatments, or behaviors. The cases and controls may be selected from a larger population or a cohort.