Does anyone know a method how to calculate the required sample size in a repeated measures design that will be analysed using linear mixed modelling? The design looks as follows:I have two groups (intervention/control) that are assessed at four time points (t0 before the intervention, t1 after the intervention, t2 after a three-month maintenance phase and t3 at 6-month follow-up). In the linear mixed model I will look at the interaction of time and group, allowing for random slopes and intercepts.
I will make the following assumptions:
Effect size for the comparison of groups at t2, adjusted for baseline values: d=0.45
variability of means: sd= 0.75
alpha level: 0.05