Purpose: This review aims to explain the definition and basic principle of statistical analysis and to clarify statistical issues related to the sample size calculation. Materials and Methods: Many formulas are available that can be applied for different types of data and study design. Results: The sample size is the number of patients or other experimental units that need to be calculated prior to the study. Determining the appropriate sample size is required to answer the research question. Conclusion: Caution is needed when applying formula for the calculation of the sample size, as it is sensitive to error and even small differences in selected parameters can lead to large differences in the sample size.