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The RelativeTimingConstraint element describes a relative timing constraint between two activities or groups of activities, represented by StudyEventGroups, StudyEvents, ItemGroups or StudyEventsItems. An in time earlier StudyEventGroup, to which the constraint applies, is referenced by the PredecessorStudyEventGroupOID.
TODO: describe similar for StudyEvent, ItemGroup and Item (i.e. PredecessorStudyEventOID, PredecessorItemGroupOID, PredecessorItemOID).
ALTERNATIVE: use "PredecessorOID" for all of these.
An in time earlier StudyEvent, to which the constraint applies, is referenced by the PredecessorStudyEventOID. An in time later StudyEventGroup, to which the constraint applies, is referenced by the SuccessorStudyEventGroupOID. An in time later StudyEvent, to which the constraint applies, is referenced by the SuccessorStudyEventOID.
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