st: comparing variables across time

[Navid Asgari <navid@nus.edu.sg>]

Hi,

I am new to the list server, though have been using Stata for the last couple of years. So, please forgive me if I am not following any of your conventions in asking a question.

I want to do a pair wise comparison of several variables across different years. I would like to know how many new characters under P1 and P2 (together)exists in year 1996 that did not exists in year 1995.



 The dataset looks like this:

  Year      P1     P2
--------  ------- ------
1995        A       B
1995        A       C	
1995        D       A
1995        E       A
1996        B       A				
1996        A       M		
1996        A	  H
1996	      A       C	  


So the code should show "2", since character M and H exist under P1 and P2 in the year 1996, but not in 1995.

Thanks in advance,
Navid









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