I understand prior to 2017 the best way to link the March basic file with ASEC data was to merge by MARBASECIDP. However, the variable is no longer available in the March basic file from 2018 forward (https://cps.ipums.org/cps-action/variables/MARBASECIDP#availability_section). I just tried to merge 2018 data using CPSIDP but there are many observations from the March basic file that did not merge to the ASEC (which surprises me, as I understood the ASEC to include all March respondents as well as some additional respondents).
What do you recommend? Thank you for the help!