REDCap: Longitudinal vs Arm

REDCap has two ways to organize repeating instruments.

Amrs are intended for e.g. control and experimental or SiteA and SiteB, but could be used as Visit 1 and Visit 2. But the preferred longitudinal paradigm is through Events. The former might be better for counting in later years, but the later keeps the data well organized (single “record ID” for entire project). REDcap will complain (but allow) if a RecordID is in more than one Arm. 20230307: NCANDA uses Arms for mutli-site (but Damara things some people exist in more than one Arm).