st: how to run nbreg with "if" command?

[sarah smith <sarah22209@gmail.com>]

Dear stata users;

I am trying to run nbreg as below;

nbreg y x1 x2 if x3==1

where x3 is a dummy variable and I want to do the regression only
using the observations where x3 is equal to 1. However I receive an
error message saying that "if is not allowed". Could you suggest a way
to do this?

Thank you very much,
Sarah
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