Controversy and Debate Series on Core Outcome Sets. Paper 1: Improving the generalizability and credibility of core outcome sets (COS) by a large and international participation of diverse stakeholders.

Core outcome sets (COSs) are agreed-on standardized sets of outcomes that should be minimally measured in all trials of a given disease. To enhance the uptake of COSs in trials, their generalizability should be increased by a change in the number and diversity of stakeholders involved in their development. Also, their credibility should be improved by methods that allow the fair capture of participants' views. This article proposes three adjustments to the development of COSs. First, instead of a qualitative study with few participants, we propose to generate the outcome domains by mapping the expectations toward treatment of a large number of stakeholders, internationally, by using an online survey with open-ended questions. Second, we propose to separate preference elicitation from the decision-making process in the selection of core outcomes. Preference elicitation would rely on an international online ranking survey, whereas the decision-making process would involve a formalized discussion among all stakeholders. Third, we propose to involve a large number of participants, including patients, in an online survey to select outcome measurement instruments. Our propositions are low-cost and scalable and help with the involvement of more stakeholders in the development of COSs, thereby increasing their generalizability, credibility, and uptake in trials.