st: about the command by

[amatoallah ouchen <at.ouchen@gmail.com>]

good day stata listers
Sorry for the question, but I''m getting stuck on some very  simple matter,
I'want to compute the number of observations with the condition  Rt=0
by country and by years

so when I' ve put
. by country  years: count if rt=0
i've got the message:  exp not allowed

whereas when I've put
by country  years: count if rt<0 and by country  years: count if rt>0
there is no trouble

I'm really puzzled

 thanks a lot in  in advance!!!!
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