Which of the following study is used for calculating Incidence rates?

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