For one of our replication projects, we have two SPSS datasets. As they are part of a follow up study (we have wave 1 and wave 3), they principally include the same cases. However, the second dataset contains some extra cases and others went missing. All cases have an identifier. We want to merge the files, but keep all of the cases of dataset 1 and add the extra cases from dataset 2. (They do differ on variables, which we also want to merge). Would anyone know how to do this?