What are the differences between the ILO's ISCO codes and IPUMS' ISCO codes?



I am attempting a crosswalk from the ISCO-68 to the ISCO-88 based on the ILO’s list of occupations at http://www.ilo.org/public/english/bur…, where the first column is the isco88 code and the second column appears to be the isco68 code. The second column codes seem to match those on the list in the isco68 document at http://www.ilo.org/public/libdoc/ilo/1969/69B09_35_engl.pdf.

I found some differences between those ILO ISCO-68 codes and the ones in the IPUMS. For one, workers with unknown occupations and those not in the universe are coded as 998 and 999 respectively in IPUMS, whereas the 2-digit code 99 (and 3-digit code 999) is reserved for Labourers Not Elsewhere Classified in the ILO sources. IPUMS instead codes Labourers Not Elsewhere Classified as 990, which does not appear in the ILO sources. My guess is that IPUMS recodes for purposes of harmonization. However, I have not found documentation on IPUMS’ recoding other than the mention of universe differences and the identification of military personnel.

Could you tell me more about the differences between the ILO codes and IPUMS’ codes for ISCO68 and ISCO88?

If it helps, I am using data from Malaysia Census 1970 to 2000.