[The research protocol V: The calculation of sample size].

Sample size refers to the number of participants to be included in a research study. If the calculation is carried out properly, not only conclusions will be established with statistical support and the potential risks to the subjects included in the investigation will be limited, but also the study can be properly planned to optimize economic and time costs. Calculation of sample size requires information that must be supported by the research protocol. This information includes that the objectives (descriptive study or a study to establish differences between groups) and hypotheses are well elaborated (with magnitude and direction), the scale of measurement of the outcome variable(s) must be defined, and type I error and type II error appropriately identified. This review describes how to specify the requirements needed for sample size calculation, including examples in clinical research designs.