Tehran University of Medical Sciences Frontiers in Emergency Medicine 2717-3593 3 1 2018 09 13 e12 e12 Nadien Fahim Faculty of Clinical Pharmacy, Zagazig University, Zagazig, El-Sharkia, Egypt Ahmed Negida Faculty of Medicine, Zagazig University, Zagazig, El-Sharkia, Egypt 2018 09 02 2018 09 02 In the previous article, we explained how to calculate the sample size for a cross-sectional study based on a rate or a single proportion. In this article, we will explain how to calculate the sample size for an independent cohort study based on a comparison of two proportions representing the event rates in both the exposed and the non-exposed groups. https://fem.tums.ac.ir/index.php/fem/article/view/104 https://fem.tums.ac.ir/index.php/fem/article/download/104/87