Power Calculations in Longitudinal Mixed Effects - from two measurements to three measurements

Side note: random effects are not the most natural way to model longitudinal data, and when you use random intercepts, adding more than around 7 repeats within subject adds no statistical information. That’s partially because random intercepts means compound symmetric correlation structure which means that time is not treated as directional. I learned this from here. The bottom line is that random effects are not likely to fit the true correlation structure.