I have a question on modelling a big data set (about 5000 subjects). I want to model a random intercept+slope, many of the subjects have only two observations (baseline and one follow-up), but some have more observations. I want to include baseline as a covariate and I am wondering whether I have to remove the observations in t=0 or if I can use them "twice" by leaving them in the data set but also using them for estimating the coefficient of baseline... hopefully my problem became clear!
The second thing concerns the scaling of the variables- I am using R and just implementing it in the formula by scale(Time), scale(Baseline) etc. However, I am a bit confused about interpreting the results, would it be better to somehow do scaling by hand in advance?
Thanks a lot!