st: About difference in differences estimation

["Lynn Lee" <lynn09v@gmail.com>]

Dear all,

When I write "diff y,t(treated) p(year)" , Stata only gives me the
difference in mean value between control and treatment groups in base year
and follow-up year respectively. But what I need is how regressors
contribute to difference in dependent variable. 

i.e. 
reg y1 x1 x2 x3
reg y2 x1 x2 x3
Then, I want to know whether x1 or x2 increases or decreases the difference
between y1 and y2.
What command I should use? 

Any suggestion is appreciated.

Best Regards,
Lynn Lee


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