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The ArmOID and EpochOID attributes may only be present when the StudyEventGroupDef is en upper-level study design building block, describing a "study cell", which is the crossing of anarm with an epoch. The value of ArmOID must be equal to the value of the OID of an Arm (element in StudyDesign). The value of EpochOID must be equal to the value of the OID of an Epoch (element in StudyDesign)StudyStructure). The ArmOID and EpochOID must be absent when the StudyEventGroupDef is referenced from another StudyEventGroupDef.