Exclusion criteria:

Information sources and search strategy.

Studies were identified by searching electronic databases and by scanning the reference lists of studies meeting the eligibility criteria, and of relevant systematic reviews. The electronic databases MEDLINE, Embase, PsycINFO, Academic Search Elite, CINAHL with Full Text and SocINDEX with Full Text and the Core Outcome Measures in Effectiveness Trials (COMET) Initiative database were searched up to June 2019. Subsequently, the search of the COMET Initiative database was updated in January 2020. Electronic searches were conducted using a combination of medical subject headings (MeSH) and relevant text word terms related to the population, in combination with different terms related to core outcome set. (For detailed information about the search strategies, Appendix S1.)

Identification of studies

Two reviewers (MÖ and CH) independently screened the titles and abstracts for eligibility. The abstracts were screened and rated using the scanning tool Rayyan, available online (7). Full text articles were retrieved and reviewed to determine eligibility, independently and in duplicate by two authors (CH and MÖ). Disagreements were resolved by discussion. The reference lists of studies meeting the eligibility criteria and of relevant systematic reviews were screened for additional relevant studies.

Description of methodology in included studies

In order to check the description of the methodology in the included studies, a checklist was compiled using the items from the COS-STAR reporting guide. One further question was added to the checklist: “Are researchers as well as healthcare providers and patients included in the development process?” (Appendix S2). Two of the authors (CH and MÖ) independently reviewed the included articles according to the checklist Disagreements were resolved by discussion.

Data items

The following information was extracted from the included trials: Population, intervention, setting for intended use, consensus method, number and characteristics of participants, number of outcomes at the start of the project and number of outcomes in the final COS, consensus criteria and the degree of compliance with COS-STAR.
Data were extracted from each included study and tabled by one reviewer. A second reviewer audited the data extraction. Any disagreements were resolved by discussion.
Since the results were not suitable for synthesis, the included studies are described narratively.