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the organization of domains into three layers is known as the “OMERACT Onion”. At the core of the Onion is the Inner Circle of mandatory domains which is split into two layers: domains that are mandatory in all trials and domains that are mandatory in specific circumstances. These should be kept to only essential domains to not overburden clinical trials. Those in the Inner Circle (note that we recommend approximately 7 (between 5 and 9) domains for the Inner Circle) are considered the ‘Core Domain Set’. In the Middle Circle of the Onion are the domains that are important but optional. In some cases, a domain might be important for certain types of trials, but not necessarily all of them. Domains needing further research go into the Outer Circle.

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