Unpacking the Cinderella black box of complex intervention development through the Partners at Care Transitions (PACT) programme of research

Introduction: Complex intervention development has been described as the ‘Cinderella’ black box in health services research. Greater transparency in the intervention development process is urgently needed to help reduce research waste. Methods: We applied a new consensus‐based framework for complex intervention development to our programme of research, in which we developed an intervention to improve the safety and experience of care transitions for older people. Through this process, we aimed to reflect on the framework's utility for intervention development and identify any important gaps within it to support its continued development. Findings: The framework was a useful tool for transparent reporting of the process of complex intervention development. We identified potential ‘action’ gaps in the framework including ‘consolidation of evidence’ and ‘development of principles’ that could bracket and steer decision‐making in the process. Conclusions: We consider that the level of transparency demonstrated in this report, aided through use of the framework, is essential in the quest for reducing research waste. Patient or Public Contribution: We have involved our dedicated patient and public involvement group in all work packages of this programme of research. Specifically, they attended and contributed to co‐design workshops and contributed to finalizing the intervention for the pilot evaluation. Staff also participated by attending co‐design workshops, helping us to prioritize content ideas for the intervention and supporting the development of intervention components outside of the workshops.