Paired and <scp>AB</scp> / <scp>BA</scp> Cross‐Over Design in Early Phase Clinical Trials: A Closer Look at Within‐Subject Variance Bias

This manuscript advocates for the implementation of multiple-sequence cross-over designs in early-phase clinical trials by investigating the bias in within-subject variance present in paired and AB/BA cross-over clinical trial designs. While the advantages of adding additional sequences to mitigate confounding effects are well established, the authors noted a lack of mathematical discussion regarding the estimation of random effects in early-phase trials-an important consideration for planning subsequent studies. The manuscript illustrates the importance of multiple-sequence designs by analysing data obtainable from a paired and AB/BA cross-over design for a normally distributed variable. It reveals that the residual mean square error from these two designs serves as an unbiased estimator of within-subject variability only under the rare conditions of no subject-by-treatment interaction and equal variances in both test and reference treatments. This implies that while paired or AB/BA cross-over design might be suitable for early pharmacological studies, it should not be relied upon solely for sample size calculations in late-stage studies due to its limited interpretative potential.