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All relationships make use of the standard domain identifiers STUDYID, DOMAIN, and USUBJID. In addition, the variables IDVAR and IDVARVAL are used for identifying record-level merge/join keys. These keys are used to tie information together by linking records. The specific set of identifiers necessary to properly identify each type of relationship is described in detail in the following sections. The following are examples of variables that could be used in IDVAR:

• When relationship datasets (e.g., SUPPxx, FAxx, CO, RELREC) relate back to split parent domains, the value of IDVAR will be from a variable with unique values for each observation. 

  1. When possible, the variable represented in IDVAR will have values from collected data (e.g., SPID, RECID), but variables with derived values may also be used (e.g., --SEQ).

• Variables from collected data
• The Sequence Number (--SEQ) variable uniquely identifies a record for a given USUBJID within a domain. The variable --SEQ is required in all domains except DM. For example, if a subject has 25 observations in the Vital Signs (VS) domain, then 25 unique VSSEQ values will be populated for the subject.
• The Reference Identifier (--REFID) variable can be used to capture an applicant-defined or external identifier, such as an identifier provided in an electronic data transfer. --REFID is permissible in all general observation-class domains, but is never required. 
• The Grouping Identifier (--GRPID) variable, used to link related records for a subject within a domain.

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