KEY_NIS contains a unique record number and not a person identifier.

KEY_NIS can be used to link NIS records from different files, such as linking records from the NIS Core and Severity files. KEY_NIS cannot be used to link to other HCUP databases (e.g. the SID).

For data years 2012-2022, the first digit of KEY_NIS identifies the Census division. Starting in data year 2023, information on Census division is excluded.

Beginning with the 2012 NIS, the data element KEY is replaced by the NIS record number (KEY_NIS).