Thank you so much for your help. Id2000 variables in both data A and data B should be matched and update Vhouse2000 variable in data B based on data A. Similarly, Vhouse2001 variable needs to be updated from data A. The problem is that even though id2000=1 and id2001=3 are in the first low together, they are different persons. I ran your code to merge two datasets. The result looks like this: id2000 vhouse2000 vstock2000 id2001 vhouse2001 vstock2001 1 100 10 3 500 50 2 200 20 4 600 60 3 300 30 70 4 40 7 700 80 5 400 50 8 800 90 6 500 60 100 Everything for year2000 (id2000, vhouse2000, and vstock2000) is good. However, two observations in id2001 are deleted (id2001 = 5 and 6) and as a result, the locations of id2001 =7 and 8 are wrong.
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