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How should I choose which ontology to use?

The ontology you choose to use depends on your local goals and local data. As long as your primary purpose is to understand and show how the i2b2 webclient works, the i2b2 demo ontology is sufficient. If your goal is to set up i2b2 for research use, one of the ACT ontologies will be more useful. In general, to set up an i2b2 instance to support  OMOP refers to a data architecture, so if you are using this locally, then ACT on OMOP will be the most relevant.

You just mentioned Domains. What do you mean by that?

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