Evidence-appraisal glossary
Protocol Deviation
Any departure from what a trial's protocol specified, such as a missed visit, a dosing error, or enrolling someone who did not fully meet the criteria. Frequent or serious deviations can distort results and signal problems in how the trial was run.
Also called: protocol violation.
Deviations vary in importance: a rescheduled visit is minor, while systematically giving one group extra care is not. How you handle them connects directly to the estimand, because whether affected patients are kept or excluded changes the question being answered. Dropping deviators to form a per-protocol set is common but risky, since it can quietly reintroduce the confounding that randomization removed.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.