Bryan Sayer, responding to a question from Nick Cox, posted:
.N is a special missing, to distinguish NIU, from don't know, refused,
invalid, etc.
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Is there a convention for special missing values to distinguish the reason for
missing values, such as .N reserved for not-in-universe (NIU)?
If so, is there a reference for an agreed-upon list of special missing values
of this nature? In a quick Google search, I could only find a limited list in
a couple of documents by Stephen Blumberg from the U.S. Centers for Disease
Control. If there's a formal convention among survey researchers, or even a
widespread informal standard, I'd rather use it than something ad hoc.
Joseph Coveney
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