I am working with CPS ASEC data and am looking at two time periods: 1978-1982 and 2015-2019. For each time period, I would like to keep only unique observations (I am interested in the cross-section of both time periods). I am therefore looking at the variable cpsidp, which identifies unique observations. However, this variable does not exist for 1978-1982. How can I now identify unique observations for this time period?
Thank you very much for your answer!