Provides information regarding levels of access, e.g., public, subscriber,
need to know. The ISO standard for dates (YYYY-MM-DD) is recommended for use
with the date attribute. More information on this element, especially its
allowed attributes, can be found in the references.
ddi_security(date = "1998-05-10",
"This variable has been recoded for reasons of confidentiality.
Users should contact the archive for information on obtaining access.")