st: Panel data manipulation

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

Dear Statalisters,

 

I’m working with a panel data and I have a little question.

I have observations for firms’ R&D expenditures for 8 years.

I would  like to keep observations for all that  firms that performs or has performed

R&D during at least one year (R&D expenditures>0 for at least one year).

How can i do this? I don’t want drop all observations with R&D==0  I want drop only firms whose R&D

expenditures has been equal to 0 for all years, and  keep all observations of  firms that

had performed R&D for at least one year, including these observations with R&D==0 for some years.

 

Thank you very much,

 

Sergio