CONCLUSION
Methodological and reporting quality of COS studies remains subject to variation. This is most frequent in the involvement of patients as research partners, outcome selection and the Delphi process. There is a clear need for further methodological research within COS development studies to determine the impact of qualitative research, the feasibility and acceptability of global COS, and attrition bias upon the degree on consensus. Clarification on these issues will help to promote the research integrity of COS development studies. The importance of implementation of COS also requires attention, including the impact and on-going relevance of the final number of core outcomes on uptake in clinical practice. Clarification on these areas will allow future core outcome set developers to actively engage with published methodological and reporting criteria and enhance the quality and ultimate success of their studies.