 This paper provides an overview of the importance of sample size and power analysis in research studies. It explains the basics of sample size calculation and power analysis, as well as the process of sample estimation and how to use G-power software for sample size and power calculations. G-power is a free software that can be used to calculate sample sizes and power for various statistical tests such as F, T, K2, Z, and exact tests. This article was authored by Hien Kong.