– Comparative vs. non-comparative

– Separate (i.e., external) control group vs. no separate (i.e., internal) control group

– Participants (study populations/groups) defined by a health outcome vs. by having been exposed to, or received or been assigned, an intervention – Prospective vs. retrospective

– Interventional vs. observational

– Experimental vs. non-experimental

– Random assignment vs. non-random assignment of patients to treatment and control groups