The per-protocol population is the group of subjects in a randomized-controlled trial that most closely stuck to their treatment regimens. As far as investigators can tell, this is the group that took their medications as directed and came to study meetings for follow-up. The per-protocol group is a smaller subset of the intent-to-treat population. In medical studies, analysis of the intent-to-treat population tends to make treatments look more similar, while analysis of the per-protocol population tends to emphasize treatment differences. When results are similar for both the intent-to-treat and per-protocol populations, it increases confidence in the study results.
