Two different invitation approaches for consecutive rounds of a Delphi survey led to comparable final outcome

Objectives: There are two different approaches to involve participants in consecutive rounds of a Delphi survey: (1) invitation to every round independent of response to the previous round ("all-rounds'') and (2) invitation only when responded to the previous round ("respondents-only''). This study aimed to investigate the effect of invitation approach on the response rate and final outcome of a Delphi survey. Study Design and Setting: Both experts (N = 188) and patients (N = 188) took part in a Delphi survey to update the core outcome set (COS) for axial spondyloarthritis. A study with 1:1 allocation to two experimental groups (ie, "all-rounds'' [N = 187] and "respondents-only'' [N = 189]) was built-in. Results: The overall response rate was lower in the "respondents-only group'' (46%) compared to the "all-rounds group'' (61%). All domains that were selected for inclusion in the COS by the "respondents-only group'' were also selected by the "all-rounds group.'' Additionally, the four most important domains were identical between groups after the final round, with only minor differences in the other domains. Conclusion: Inviting panel members who missed a round to a subsequent round will lead to a better representation of opinions of the originally invited panel and reduces the chance of false consensus, while it does not influence the final outcome of the Delphi. (C) 2020 The Authors. Published by Elsevier Inc.