This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'Which of the following study is used for calculating Incidence rates? \nSelect and explain. \nA. Coho study, B. Cross over study, C. Case control study, D. Cross sectional study', 'role': 'user'}, {'content': "Answer: A \nExplanation: In a coho study we can determine incidence rates directly in those exposed and those not exposed. Incidence Rate is defined as the number of new cases occurring in a defined population during a specified period of time. * Incidence rate = Number of cases in a defined period/mean number of persons exposed to risk in that period * 1000 Ref: Park's Textbook of Preventive and Social Medicine, By K. Park, 19th Edition, Pages 56, 70, 73, 74; Evidence-Based Practice in Nursing and Healthcare: A Guide to Best Practice By Bernadette Mazurek Melnyk, Section 4; Introduction to Public Health By Mary-Jane Schneider, 3rd Edition, Pages 74-5", 'role': 'assistant'}]