"Vicarious thinking" was a key driver of score change in Delphi surveys for COS development and is facilitated by feedback of results

Objective: The objectives of this nested study were to (1) assess whether changes in scores between rounds altered the final degree of consensus achieved in three Delphi surveys conducted as part of COS development projects (anal, gastric, and prostate cancer), and (2) explore participants' reasons for changing scores between rounds.

Study design and setting: All Delphi surveys were conducted online using DelphiManager software and included healthcare professionals and participating patients. Participants were invited to give a free-text reason whenever they changed their score across an important threshold on a 1-9 Likert scale (1-3 not important, 4-5 important, 7-9 critically important). Reasons for score change were coded by four researchers independently using an inductive-iterative approach.

Results: In all three Delphi surveys, the number of outcomes reaching criteria for consensus was greater in R2 than R1. Twelve themes and 23 subthemes emerged from 2298 discrete reasons given for score change. The most common reasons for the change were "time to reflect" (482 responses, 23%) and vicarious thinking (424, 21%), with 68% (291) of vicarious thinking attributed to seeing other participant's scores.

Conclusion: Our findings support conducting a Delphi survey over the use of a single questionnaire where building consensus is the objective. Time to reflect and vicarious thinking, facilitated by seeing other participant's scores, were important drivers of score change. How results are presented to participants between rounds and the duration of and time between rounds in a Delphi survey may, therefore, influence the results and should be clearly reported.

Keywords: Core outcome set development; Delphi survey; Feedback methods; Stakeholders; consensus; patient public involvement.