Cohort Studies: These follow a group of people over time to observe how various exposures affect the incidence of diseases. Case-Control Studies: While typically shorter than cohort studies, case-control studies can also be designed to look retrospectively over long periods. Randomized Controlled Trials (RCTs): Long-term RCTs are less common due to cost and logistical challenges but are invaluable for understanding the long-term impact of interventions.