Principles of Sample Size Calculation

Sample size calculation is the process of determining the appropriate number of participants to include in a clinical trial. The size of the sample should be adequate, allowing statistical analyses to show relevant treatment effects and to generate conclusive results. The larger the number of participants in a trial, the more reliable the conclusions will be. However, larger studies require more resources (both in terms of finances and patient commitment), and may increase the risk for participants to be exposed to inefficient or even unsafe treatment. It is therefore important to optimize the sample size. Moreover, calculating the sample size in the design stage of the study is increasingly becoming a requirement when seeking a favourable ethics committee opinion for a research project.

The wide range of formulas that can be used for specific situations and study designs makes it difficult for most investigators to decide which method to use. The calculation of the sample size is troubled by a large amount of imprecision, because investigators rarely have good estimates of the parameters necessary for the calculation. Unfortunately, the required sample size is very sensitive to the choice of these parameters and small differences in selected parameters can lead to large differences in the sample size