On a related note, learned today about a parallel (and much shorter) concept paper from the EMA outlining their planned approach to developing guidance on Bayesian methods in clinical trials. The public comment period closes April 30, 2026 and those interested can submit via EUSurvey.
Given our ongoing discussion of the FDA draft guidance here, this provides a useful comparison point and an opportunity for the statistical community to provide input to EMA. From my understanding (correct me if I am wrong), the EMA document is not meant to be a full guidance but rather a description why EMA thinks additional guidance is needed and what topics/questions the future reflection paper should tackle.
In contrast to the FDA document, the EMA framing suggests Bayesian methods require special justification rather than being recognized as a coherent inferential framework. The emphasis on “error control” and “lack of control of type I error rate” throughout the document suggests the EMA is still viewing Bayesian approaches through a fundamentally frequentist lens. This contrasts sharply with the FDA draft guidance’s recognition that there are legitimate Bayesian operating characteristics (probability of correct decisions, Bayesian power, expected bias and MSE of estimates averaged under a prior) that do not reduce to α-control.
For example, the EMA paper asks: “How to assess error control for both primary and secondary endpoints in the absence of frequentist inference?” and “How to deal with lack of control of type I error rate?” The FDA guidance, by contrast, explicitly states that calibration to Type I error rate “may not be applicable or appropriate” in Bayesian settings and provides detailed discussion of alternative approaches to specifying success criteria.
The proposed timeline extends to June 2028 for the final reflection paper representing a significant lag behind current FDA thinking and the broader methodological literature. Given that EMA is seeking feedback, this may be an opportunity to encourage alignment with the more sophisticated treatment in the FDA guidance. Specifically:
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Recognition that Bayesian methods do not require special justification when the design and analysis are coherent.
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Acknowledgment of Bayesian-specific operating characteristics beyond Type I error.
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Guidance on prior distributions beyond just informative priors for borrowing.
Additional thoughts welcomed. Europe has exceptional statisticians and clinicians who have made foundational contributions to Bayesian methodology in clinical trials. Additionally, the existence of the FDA draft guidance may serve as a catalyst, providing both a template and external pressure for the EMA’s thinking to evolve. Particularly since the concept paper is a starting point for consultation rather than a final position.