st: drop command & missing values

["Francesco Trivieri" <francesco.trivieri@unical.it>]

Dear Maarten and Martin,

thank you very much for your answers. I am always including the x variable 
in my estimation, I don't understand why:
drop if x>123
xtabond2    Y   Y_1    X    Z ....
the estimation sample is 8100

drop if x>123 & x!=.
xtabond2    Y   Y_1    X    Z ....
the estimation sample is 12200

thanks again
Francesco 

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