ODM defines a system of OID uniqueness constraints for Element Identifiers and references, which only require uniqueness within certain contexts. For example, the Study OID has to be unique within a series of ODM documents, while an OID for ItemGroup has to be unique within its Study only. Aside from that, notable requirements for ODM OID uniqueness include the following:
- ODM OIDs for each element type inside a MetaDataVersion must be unique for that scope of the MetaDataVersion.
- MetaDataVersion OIDs must be unique within the containing Study.
- ArchiveLayout OIDs, while not used in OpenClinica yet, must be unique within a single FormDef.
- Study OIDs must be unique within the containing ODM.
- Measurement Unit OIDs must be unique within the containing Study.
- User, Location and Signature OIDs, while not used in OpenClinica yet, must be unique within the containing AdminData.
OpenClinica generally adheres to these contraints, but OpenClinica OIDs are also unique across all studies in a single instance of OpenClinica. Because of this, an entity (such as a CRF) loaded into a separate instance of OpenClinica may not have the same OIDs in the new instance since those OIDs may already be assigned. This has drawbacks for reuse of CRFs and Rules across instances (such as porting a study definition from test to production) because the OIDs of items, groups, forms, and events that are referenced in the Rules may change when loaded into the new instance.