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Here is another screenshot, where you can see the expansion for "Race" in the "Demographics" tree:

Where

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do i2b2 ontology "concepts" come from?

Typically, the concepts that make up the various categories or "domains" are based on medical terminology standards published by various institutions. Concepts can also be created ad hoc for a specific i2b2 instance by the i2b2 administrator at that site. (For more information on custom metadata, please see Ontologies 201.)

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