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

In interventional studies, participants are assigned to receive one or more interventions (or no intervention) so that researchers can evaluate the effects of the interventions on biomedical or health-related outcomes. Arm refers to each group or subgroup of participants in a clinical trial that receives specfic interventions (or no intervention) according to the study protocol. This is decided before the trial begins.

In observational studies, participants may receive diagnostic, therapeutic, or other types of interventions, but the investigator does not assign participants to specific interventions (as in an interventonal study.) Group or cohort refers to participants that are identified as belonging to a study group that is assessed for biomedical or health outcomes.