As mentioned in this forum post, the IPUMS CPS 2001 ASEC sample includes cases from the S/CHIP oversample, which are not included in the original 2001 ASEC data file on the Census Bureau’s website. You can access both files as well as the original codebook through this NBER data page. This IPUMS working paper (Rodgers, 2025) provides additional details about the oversample, which added approximately 20,000 households to the ASEC.
Regarding the 1988 rewrite of the CPS processing system, I can confirm the IPUMS CPS 1988 ASEC sample uses the redesign file that was created by applying the new processing system. The NBER data page includes a note explaining that the sample sizes of the two files are different due to differing eligibility requirements. It also includes documentation and data files for both versions.
Moving to the pre-1976 samples, I’m not sure exactly what would cause a discrepancy for these samples. One possibility is that for samples prior to 1980, the age cutoff for being in-universe for OFFPOV is 14 years (anyone below the age of 14 and not in FTYPE = 5 “secondary individual” is not-in-universe for OFFPOV). In 1980, this age cutoff increases to 15 years. If you could share more about your analysis and the mismatches you’re seeing, I can share more information on what may be happening.