st: Creating new variable from existing observations

[Suryadipta Roy <sroy2138@gmail.com>]

Dear Statalisters,

I am trying to create the last column (i.e. the variable M_imp_new)
where the data take the following form:

Xcode	Mcode	YEAR	X_exp	M_imp	X_Treat	M_Treat	M_imp_new
CAN	MEX	1990	2	8	1	0	8
CAN	MEX	2000	3	9	1	1	8
CAN	MEX	2010	4	9	1	1	8
MEX	CAN	1990	8	2	0	1	2
MEX	CAN	2000	9	3	1	1	2
MEX	CAN	2010	9	4	1	1	2
US	CAN	1990	2	2	0	1	2
US	CAN	2000	1	3	0	1	2
US	CAN	2010	3	4	1	1	2
US	MEX	1990	2	8	0	0	8
US	MEX	2000	1	9	0	1	8
US	MEX	2010	3	9	1	1	8
MEX	US	1990	8	2	0	0	1
MEX	US	2000	9	1	1	0	1
MEX	US	2010	9	3	1	1	1
CAN	US	1990	2	2	1	0	1
CAN	US	2000	3	1	1	0	1
CAN	US	2010	4	3	1	1	1

The M_imp_new is supposed to take the value of M_imp before M-treat
changes from 0 to 1 for each country. E.g. for MEX, M-imp = 8 in
year==1990 (M-treat=0) and then MEX experienced the treatment such
that M_Treat = 1 in year==2000. Thus, M_imp_new= 8 for MEX. I have
been struggling with this for some time without success, and would
sincerely appreciate some help.

Thank you very much in anticipation!
Suryadipta.
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