> 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 t

> 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. It’s common for surveys to encode various null values. ‘Refused to answer’, ‘Did not know’, ‘Not applicable’, ‘Illegible response’ etc, although there is no standard way to do this. The SAS language allows you to use multiple custom missing values, but this is not widely known and I’ve never seen anyone do it.