(A) Useful for rare diseases # CASE-CONTROL STUDY: Both exposure & outcome have occurred before the start of the study the study proceeds backwards from effect to cause; & it uses a control or comparison group to support or refute an inference.ADVANTAGES AND DISADVANTAGES OF CASE CONTROL STUDIESADVANTAGESDISADVANTAGES1. Relatively easy to carry out1. Problems of bias relies on memory or past records, the accuracy of wh'ch may be uncertain; validation of information obtained is difficult or sometimes impossible2. Rapid and inexpensive (compared with cohort studies)2. Selection of an appropriate control group may be difficult3. Require comparatively few subjects3. We cannot measure incidence, and can only estimate the relative risk4. Particularly suitable to investigate rare diseases or diseases about which little is known. But a disease which is rare in the general population (e.g., leukaemia in adolescents) may not be rare in special exposure group (e.g. prenatal X-rays).4. Do not distinguish between causes and associated factors5. No risk to subjects5. Not suited to the evaluation of therapy or prophylaxis of disease6. Allows the study of several different aetioiogical factors (e.g., smoking, physical activity and personality characteristics in myocardial infarction)6. Another major concern is the representativeness of cases and controls7. Risk factors can be identified. Rational prevention and control programmes can be established 8. No attrition problems, because case control studies do not require follow-up of individuals into the future9. Ethical problems minimal