This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
In case control studies 
Select and explain. 
A. Many patients are needed, B. Incidence can be calculated, C. Cause to effect is preceded, D. Odd ratio can be calculated
D: Odd ratio can be calculated
Answer: D: Odd ratio can be calculated 
Explanation: .case control studies are retrrspective studies,that proceeds from effect to cause.it requires two study groups,namely cases and controls.both exposure and outcome has occurred at the time of study.they are basically comparison studies,four basic steps of this incudes, selection of cases and controls matching measurement of exposure analysis and interpretation,from a case control study we can derive odd&;s ratio that is a measure of association between risk factor and outcome.it is closely related to relative risk. ref:park&;s textbook,ed22,pg no 70