Purpose: The purpose of this study was to introduce the main concepts of statistical testing and effect size and to provide researchersin nursing science with guidance on how to calculate the effect size for the statistical analysis methods mainly used in nursing. Methods: For t-test, analysis of variance, correlation analysis, regression analysis which are used frequently in nursing research, thegenerally accepted definitions of the effect size were explained. Results: Some formulae for calculating the effect size are describedwith several examples in nursing research. Furthermore, the authors present the required minimum sample size for each exampleutilizing G*Power 3 software that is the most widely used program for calculating sample size. Conclusion: It is noted that statisticalsignificance testing and effect size measurement serve different purposes, and the reliance on only one side may be misleading. Some practical guidelines are recommended for combining statistical significance testing and effect size measure in order to makemore balanced decisions in quantitative analyses.