In each group, we
used a third-order polynomial (linear, squared, and cubic
terms) of time to model the probability of being adherent.
On the basis of these models, we assigned patients to a trajectory
group. We implemented this model with “PROC
TRAJ,” a free downloadable add-on package to base SAS
version 9.3 (SAS Institute, Cary, North Carolina). This
estimation procedure has been shown to be superior for
identifying underlying longitudinal trajectories