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.