Sample size calculation.

When designing any research project, definition is required of the sample size needed in order to carry out the study. This sample size is an estimate of the number of patients required in accordance with the pursued study objective. In this context, it is more efficient in terms of both cost and time to use samples than to work with the entire population. The present article describes the way to establish sample size in the kinds of studies most frequently found in health research, and how to calculate it using the epicalc package included in the shareware R program. A description is provided of the formulae used to calculate sample sizes for the estimation of a mean and percentage (referring to both finite and infinite populations) and for the comparison of two proportions and two means. Likewise, examples of the application of the mentioned statistical package are provided.