Retrospective study is also known as-

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 factorCases(disease present)Control(disease absent)PresentAbsentabcda + 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.