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
Which of the following study is used for calculating Incidence rates? 
Select and explain. 
A. Coho study, B. Cross over study, C. Case control study, D. Cross sectional study
A: Coho study
Answer: A: Coho study 
Explanation: 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