Evidence-appraisal glossary
Per-Protocol Effect
The treatment effect you would see if everyone actually followed the trial's protocol as intended. It is a target of estimation, distinct from the per-protocol analysis that naively tries to measure it.
Also called: as-directed effect.
The per-protocol effect answers a question patients and clinicians often care about: what does this treatment do when it is actually taken as directed? The obvious approach, analyzing only the participants who adhered, breaks randomization and invites confounding, because people who stick with a treatment differ from those who do not. Estimating it credibly usually requires methods that adjust for the reasons people deviated, such as inverse probability weighting.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.