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The Comments (CO) special-purpose domain is used to capture unstructured free-text comments. It allows for the submission of comments related to a particular domain or those collected on separate general-comment log-style pages not associated with a domain. Comments may be related to a subject, a domain for a subject, or to specific parent records in any domain. The CO special-purpose domain is structured similarly to the Supplemental Qualifiers (SUPP--) dataset, in that it uses the same set of keys (STUDYID, RDOMAIN, USUBJID, IDVAR, and IDVARVAL) to identify related records.

All comments except those collected on log-style pages not associated with a domain are considered child records of subject data captured in domains. STUDYID, USUBJID (or POOLID), and DOMAIN (with the value CO) must always be populated. RDOMAIN, IDVAR, and IDVARVAL should be populated as follows:

• Comments related only to a subject in general would have RDOMAIN, IDVAR, IDVARVAL null, as the only key needed to identify the relationship/association to that subject is USUBJID (or POOLID in the case of comments on a pool of subjects).
• Comments related only to a specific domain (and not to any specific record(s)) for a subject would populate RDOMAIN with the domain code for the domain with which they are associated. IDVAR and IDVARVAL would be null.
• Comments related to specific domain record(s) for a subject would populate the RDOMAIN, IDVAR, and IDVARVAL variables with values that identify the specific parent record(s).

Additional information may be placed in COREF to further describe the comment's relationship to the parent record. This can only be done if the relationship cannot be represented using the variables RDOMAIN, IDVAR, and IDVARVAL.

For clinical studies only:

• Values (e.g., CRF page number or name) may be placed in COREF.
• Timing variables (e.g., VISITNUM, VISIT) may be added to the CO special-purpose domain. See Section 5.1, Comments, assumption 5 for a complete list of identifier and timing variables that can be added to the CO special-purpose domain.

As with Supplemental Qualifiers (SUPP--) and Related Records (RELREC), --GRPID and other grouping variables can be used as the value in IDVAR to identify comments with relationships to multiple domain records. The limitation of this is that a single comment may only be related to a group of records in 1 domain (RDOMAIN can have only 1 value). If a single comment relates to records in multiple domains, the comment may need to be repeated in the CO special-purpose domain to facilitate the understanding of the relationships.