a conceptual framework used to organize and prioritize outcome domains for Core Domain Sets in rheumatology and musculoskeletal research. It is visualized as three concentric circles (layers), representing the level of priority and evidence for each domain:
Inner Circle: Mandatory core domains required in all trials. This layer is kept intentionally small (recommended 5–9 domains) to avoid overburdening studies.
Middle Circle: Domains that are mandatory only in specific circumstances or for certain types of studies.
Outer Circle: Domains on the research agenda—candidate domains requiring further validation or investigation before potential inclusion in the core set.
The Onion provides a shared language and structure for consensus-building, discussions, and voting when developing or updating Core Domain Sets.