I have created a Shiny app to calculate the required sample size for a two-group linear mixed model.
It uses the
power.mmrm function from the
longpower package (see Lu, Luo, & Chen, 2008, for more information).
The input is simplified, whereby users are limited to five time-points, the correlation matrix is a compound symmetry correlation structure that is identical for both groups, and the attrition rate is assumed to be the same for both groups.