Hi Chris, Thank you for your response. I just want to append (stack) one on another. Here is an example. The first input data set is NLSY79 that has 25 original variables. The second input data set is NLSY97 that has 25 original variables. So I would run: Data NLSY79andNLSY97; Set NLSY79 NLSY97; The output data is NLSY79andNLSY97. However, after combining these 2 data sets, I'd like to run a logistic model on both data set with a interaction term to be added on NLSY97 only. So I need to give SAS a signal that add an interaction term on NLSY97 only and not on both. That is why I need dummy variables that identify NLSY97 before I combine it with NLSY79. But I don't know who to make those dummy variables on each data set. I hope this make it clearer. If not, I can explain more. Thanks.
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