NIU vs. unknown for Industry variables

I am working with “ind” , “ind1990” , “occ” , “occ2010” variables in CPS. I noticed that there are no observations coded as “unknown”. But the only observation with missing industry/occupation information are those coded as “NIU”. How are individuals that don’t respond to industry and occupation questions coded? Are they coded as a part of “NIU” ?

In the CPS, individuals who do not give a response for a given question generally have the value imputed by the Census Bureau. You can identify which cases have imputed values by looking at the variable flags. The documentation for each variable has a section listing any flags for that variable, for example the flag for OCC is listed here.