It seems that those unemployed are classified as 999 (Unknown) in OCC90LY. I say that because when I look into another occupation variable, OCC10LY, for the same observations, most are classified as 9920 (Unemployed, with No Work Experience in the Last 5 Years or Earlier or Never Worked).
Why are the unemployed classified as 999 in OCC90LY? is it safe to assume that those 999 in OCC90LY are actually unemployed?
The various non-occupation codes for the “occupation last year” variables in ASEC (0 for OCCLY, 999 for OCC50LY, 999 for OCC90LY, 9920 for OCC10LY) should all be interpreted as NIU codes. The values and labels for codes that do not report occupations differ for each of these variables; however, the number of people with an occupation code in each of these variables is the same. In response to your specific question about interpreting OCC90LY values of 999 as unemployed, I would frame the variable as reporting occupation codes and non-occupation codes. If you are interested in people who are unemployed, I suggest using EMPSTAT. Thank you for pointing out these discrepancies. The CPS team will work on standardizing the NIU codes and universe statements, which will be completed in a future data release.