<prompt> In a case control study, the most characteristic feature is -Odds ratio estimationProblem biasYield incidence rateExpensive  <response>: Ans. is a i.e., Odds ratio estimation o Final step in case control study is analysis to find out ? i Exposure rates among cases and controls to suspected factor. ii Estimation of disease risk associated with exposure odds ratio. i Exposure rates o A case control study provides a direct estimation of the exposure rates frequency of exposure to a suspected factor. ii Odds - ratio cross product ratio o From a case control study odds ratio can be derived which a measure of the strength of association between risk factor and outcome. o Odds ratio is closely related to relative risk which is analysed in coho study -- odds ratio represents only an estimate of relative risk. o The odds ratio is cross product of entries in above table o Odds ratio is a key parameter in analysis of case control studies. Why is it not possible to estimate relative risk in case-control study ? o Relative risk is the ratio between the incidence of disease among exposed and incidence among non-exposed. o So, for relative risk estimation, incidence rate is required. o But incidence rate can not be calculated by case control study as there is no denominator poulation at risk. o Because both disease and exposure have already occured, there is no risk for the subjects in the study -- There is no population at risk. 