st: Keep observations in panel data

[s <s.afcha@ub.edu>]

Dear Statalisters


I have a panel data with the follow structure:

id year subsidy

1 2000   0
1 2001   0
1 2002   1
1 2003   0
1 2004   0
1 2005   1
2 2000   0
2 2001   0
2 2002   1
2 2003   1
2 2004   0
2 2005   1
32000    1
32001    1
32002    1

The data describe reception of subsidy from 2000 to 2005. My purpose is 
evaluate
the subsidy effect in the two years subsequent to the reception of 
funds. Then I want only those firms
with subsidies in T but without subsidies in t-1, t+1 and t+2. 
Consequently, in the previous
example I will keep  only the observations of firm 1 in the period 
2001-2004.
How can I do this?


Thanks

S.
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