| Sumario: | Calculation of exact sample size is an important part of research design. It is very important to understand that different study design need different method of sample size calculation and one formula cannot be used in all designs. In this article different formulae of sample size calculations are explained based on study designs. In clinical research, sample size calculation plays an important role for assuring validity, accuracy, reliability, and integrity of the intended clinical study. For a given study, sample size calculation is usually performed based on some statistical criteria controlling type I error or type II error. To estimate sample size power analysis is probably the most commonly used method for sample size calculation. We will focus on sample size calculation based on power analysis for various situations in clinical research.
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