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
[{'content': 'In a case control study, the most characterstic feature is - \nSelect and explain. \nA. Odds ratio estimation, B. Problem bias, C. Yield incidence rate, D. Expensive', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Odds ratio is a measure of the strength of association between risk factor and outcome (refer pgno:73 park 23 rd edition)', 'role': 'assistant'}]