Limitations and future work
Our simulation study considered a wide range of scenarios and mean trajectories. However, as always with a simulation study, these cover a small fraction of all possible choices. For example, we chose to only consider 80% power. At 90% power trial sample sizes will be larger, and we anticipate that this will provide more protection against violation of assumptions due to the central limit theorem.In addition, we considered only one size of observational study. We anticipate that varying the size of the observational study is only likely to affect the results through introducing more (or less) uncertainty in the model estimates from the observational study (these then being used in the sample size calculation). We chose a large sample size of 1000 participants, in order that those estimates are relatively precise, but that the size of the dataset is still in line with datasets that may be available for trial planning. Using a smaller sample size for the observational study would result in larger Monte Carlo errors but not alter patterns of results. We chose to look at the impact of increasing the uncertainty by including scenarios with a larger residual error variance instead.Under certain circumstances investigators may want to consider using the free trajectories, free covariance model in the sample size calculation. The methodology outlined in Frost et al. [