Causality timing and study direction
Cohort studies flow forward from exposure to outcome, mimicking real-world causality: exposed people are tracked to see if disease develops. Case-control studies flow backward: people with disease are identified, then past exposure assessed. This temporal directionality gives cohorts a causal advantage; they naturally answer 'does this exposure lead to disease?' while case-control studies demand careful interpretation to avoid reverse-causality arguments.
Cohort studies measure relative risk (probability of disease given exposure); case-control studies measure odds ratios (odds of exposure given disease). The statistics differ in interpretation, though they approximate each other when disease is rare.
Efficiency and feasibility trade-offs
Cohort studies suit common exposures or outcomes; case-control suits rare diseases. For a rare disease affecting 1 in 100,000 people, a cohort study would need to enroll hundreds of thousands and follow them for years, becoming infeasible. A case-control study identifies existing cases and controls, requiring far fewer participants and no follow-up time.
Cohort studies cost more and take longer but generate richer causal evidence. Case-control studies are cheaper and faster but vulnerable to recall bias and selection bias. The choice depends on disease frequency, available resources, and causal evidence strength needed. Many epidemiological questions begin with case-control hypotheses, then confirm with cohorts when feasible.