Which statement best defines per-protocol analysis?

• A. Analyzes outcomes only for participants who completed the study according to protocol.
• B. Randomizes participants after enrollment.
• C. Includes all randomized participants irrespective of adherence.
• D. Measures outcomes using intention-to-treat principles.

Answer: (A)

Per-protocol analysis focuses on participants who completed the study exactly as planned, analyzing only those who adhered to the protocol. This approach aims to estimate the effect of the intervention under ideal conditions, reflecting what happens when protocol adherence is followed. That’s why describing outcomes for participants who completed the study according to protocol best fits per-protocol analysis.

The other ideas describe different concepts: randomizing after enrollment speaks to when randomization happens, not how the analysis is conducted. Including all randomized participants irrespective of adherence aligns with intention-to-treat, which preserves randomization and assesses effectiveness in a real-world, nonadherent scenario. Similarly, measuring outcomes using intention-to-treat principles points to ITT rather than per-protocol.