There are two kinds of keys in the ODM: internal and external. Internal keys are used to designate entities within the model, and to allow cross-references between entities within (and between) ODM files. Internal keys are not for human use, and are assumed to be unchanging. Internal keys include subject keys, element OIDs, and repeat keys. To fully identify a clinical data entity, the following internal keys are needed:
| Kind of Entity | Identifying Keys | | study | StudyOID | | subject | above plus SubjectKey | | study event | above plus StudyEventOID and StudyEventRepeatKey | | item group | above plus ItemGroupOID and ItemGroupRepeatKey | | item | above plus ItemOID | | annotation | keys for the annotated entity plus SeqNum |
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