@FrankHarrell Thank you for your answer. If I use MCMC (I'm using R2Openbugs), can I just take the sampled values of mu and sigma (after the burn-in period) and calculate for each pair the corresponding quantile?

The different trajectories of the possible outcomes.

Thank you for your reply. But then, isn't it always better to use a random slope model instead of using a paired t-test/random intercept model? In reality, every subject will have a different slope, no? If you use the paired t-test, there is no assumption that the variances in the two groups are equal, but using the random intercept model, there is.