- investigating a hypothesis about the cause of disease by studying how exposures relate to disease ➔ Observational (cohort, case control, cross-sectional, ecologic study) - researcher observes association between exposure and disease, estimates and tests it ➔ Experimental (RCT, quasi experiment) - researcher assigns intervention (treatment), and estimates and tests its effect on health outcome. -Used to search for causes and effects, or the WHY & the HOW.