I once had a statistics professor (teaching a programming class using, IIRC, SAS) in college who worked as a data analyst in the federal government, possibly the Labor Department. Part of his job was working with polling and form data. After class he once tried to explain a paper he wrote arguing for an additional null-like type in the formal analytics system in his domain. I was young and stupid (took it as an easy programming course; was not skilled at abstract math) and impatient (wanted to meet up with friends at a pub) so didn't follow along well enough to even properly remember his argument, let alone understand it. But I've always distinctly recalled that he was arguing for a 4th, or possibly 5th type, yet whenever I survey the literature I've only found at most such 2 types. But I've probably been looking in the wrong places.

For context, NULL/missing was a type. N/A was a type (i.e. a question irrelevant to someone). I can't remember the 3rd, and of course not the additional (4th or 5th) type he was proposing. If anyone knows a good resource that describes this problem domain, that would be much appreciated. I've been haunted for over 20 years by the regret of not fully appreciating what he was trying to explain to me, both because I could've learned something that day, but also because I almost immediately felt like an a*hole for not showing enough interest in and respect for a piece of scholarly work he was clearly proud of and wanted to share with me.

EDIT: I think the third pre-existing type was for a situation where someone abstains from answering a question like, "Sex: male/female", because they consider themselves neither. So something like an Other type. In retrospect this was presumably related to set theory, and he felt there was a gap in the pre-existing formal models.