The sample size justification is a fundamental statistical section in the study protocol that explains in detail how the number of participants required for a study (the sample size) was calculated. A study with too few participants may not have the statistical power to demonstrate a clinically relevant treatment effect, even if one exists. A study with too many participants is uneconomical and unnecessarily exposes too many individuals to the risks of an investigation. Therefore, the correct calculation of the sample size is an ethical and scientific imperative.
The calculation is based on several factors: the primary endpoint, the expected magnitude of the treatment effect, the variability of the data (standard deviation), the desired significance level (alpha error, typically 5%), and the target statistical power (usually 80% or 90%). For CRO biostatisticians, preparing a scientifically sound and transparent sample size justification is one of the most important tasks in study design, as it forms the basis for resource planning and the overall project’s chances of success.