Thank you for your help! When I try your code, the number of observations increased (not something I expected). What happened was that for each patid that appears in 'allfills', the number of observations for that individual in 'nif2' is the number of obs in 'nif1' multiplied by the number of obs in 'allfills'. Here's a made-up example of what I'm trying to get: nifgroup.nif1 patid fill_dt 100100 08/05/2019 100100 09/01/2019 100100 09/30/2019 100100 10/28/2019 100100 11/29/2019 amlgroup.allfills patid fill_dt 100100 01/02/2019 100100 03/30/2019 100100 06/25/2019 100100 07/18/2019 nifgroup.nif2 patid fill_dt amlinwash 100100 08/05/2019 1 100100 09/01/2019 1 100100 09/30/2019 1 100100 10/28/2019 1 100100 11/29/2019 0 For patid 100100, this person currently has 20 observations in nif2, with each fill_dt repeating 4 times. I need the individual to retain the original number of observations from 'nif1' when creating 'nif2', with only the creation of amlinwash added to the new dataset (as shown in table). In my data, each fill_dt has a unique 'fill ID'; if I take the first unique value of the fill ID within each unique patid, I should get what I need.
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