Accuracy of the assumptions formulated for sample size calculation of randomized controlled trials and study of the associated factors

Sample size calculation is a determining step of the design of a randomized controlled trial. Its role is to determine the minimal number of participants to include in the study, in order to detect a meaningful, pre-defined, therapeutic effect. It shouldn't be either under or over-sized. Its calculation is based on : error rates (type 1 and 2), assumptions for the value of the primary outcome in the control group, and the minimal clinically meaningful difference to be detected between experimental and control groups. These parameters are based on predictions and so can be far from the observed value in the trial. In these cases, the error risks are not controlled to the desired levels anymore, which can complicate the final interpretation of the trial. We aim to actualize the findings of a previous work published 17 years ago, analysing a cohort of superiority randomized controlled trials published between 2023 and 2025, to evaluate whereas there has been a progression in the sample size reporting and in the accuracy of sample size assumptions. We also aim to identify the trials and sample size characteristics that could be associated to accuracy of assumed and observed values of the control group. We expect to find a notable difference between assumed and observed values, which decreased over the years. We also expect that some characteristics of the trial and the sample size calculation are associated with the accuracy of the assumptions