I´m currently conducting a randomized placebo trial where we test the effect of one treatment on a single outcome variable. We measure the outcome variable at four time points for both groups: baseline, 3 months, 6 months and 12 months.

There is no crossover in the trial and we expect the largest part of the difference (if any) between the two treatment to occur between baseline and the first measurement at three months.

Which statistical test would be most suitable to used to show that there is a difference between the treatment and placebo?

I´m pretty used to working with STATA, but completely new to repeated measure analysis - so any thoughts and explanations are gratefully appreciated.