Sample size determination for randomized decentralized clinical trials with longitudinal outcomes

Decentralized clinical trials (DCTs) offer significant advantages in enhancing trial accessibility, reducing patient burden, and improving population representativeness. These benefits are particularly pronounced in longitudinal trials, where patient outcomes are measured repeatedly over time. However, existing literature lacks appropriate sample size formulas tailored to longitudinal DCTs, which must account for heterogeneous measurement sources and increased uncertainty introduced by decentralized data collection. We propose a unified sample size calculation method for randomized longitudinal DCTs under the generalized linear model framework. The proposed formula is available in closed form and flexibly accommodates a variety of data collection designs, including the simple hybrid design, where all patients contribute both onsite and offsite data, and the stratified hybrid design, where different patient groups may undergo distinct data collection protocols. We also develop estimation procedures to estimate the unknown parameters and compute the sample statistics. Our method further generalizes to accommodate heterogeneous intraclass correlation coefficients, unequal variances between treatment arms, and varying numbers of data collection types or strata. Extensive simulation studies show that the proposed sample size formula achieves robust control of the type I error rate and maintains the desired power across a wide range of realistic scenarios.