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st: diff-in-diff in levels or in logs


From   Jack Knife <olaf.collider@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: diff-in-diff in levels or in logs
Date   Thu, 2 May 2013 15:18:19 +0200

Dear all,

I am estimating a difference in difference regression with stata...
and I would like to know what would be the best approach:
I estimate a difference in difference in levels: that is if my output
variable is Y, treated individuals have the dummy D=1 and treatment
period is in time T=2

I run -reg Y T D T#D

and the coefficient on T#D gives me the diff in diff estimates. But if
Y is a count variable... how could I say that Y decreased by X% after
the reform using the same approach?
Should I regress log(Y) instead of Y?

Many thanks for your help

-Jack
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