<prompt> Retrospective study is also known as-Case control studyCross-sectional studyCohort studyExperiment studies  <response>: Ans. is a i.e., Case control study Case control study is a common first approach to test causal hypothesis. Case - Case is an individual who has developed the disease. Control - Is an individual who has not developed the disease but is otherwise comparable to case e.g. age, sex, occupation, social status etc. The case - control study has three distinct features -i Both exposure risk factor and outcome disease have occurred before the start of study - so, there is no further risk for subjects.ii Study proceeds backwards from effect to caused retrospective study.iii It uses a control or comparison group to support or refute an inference. The focus is on a disease that has already developed. Association between risk factor and disease can be tested - risk factor can be identified. For example, if it is our intention to test the hypothesis that cigarette smoking causes lung cancer, the investigation begins by assembling a group of lung cancer cases a b and a group of suitable matched control not having lung cancer. Then past history for the presence or absence of smoking is explored in both groups.Suspected risk factorCasesdisease presentControldisease absentPresentAbsentabcda cb d If the frequency of smoking, a a c is higher in cases than in control b b d, an association is said to exist between smoking and lung cancer. 