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Re: st: DiD with panel data

From   Austin Nichols <>
Subject   Re: st: DiD with panel data
Date   Thu, 19 Feb 2009 12:28:12 -0500

Fiorenzo Bovenzi <>:
It all depends on what you have in mind as a D-in-D model. In something like
 egen id=group(id_var country)
 xtreg Yist Dist, i(id) fe
the coef on Dist compares pre-treatment values to post-treatment
values within firm-country pairs; firm outcomes in countries that
never have Dist=1 contribute nothing to the estimate, but I think you
want to use a firm's outcomes in such a country as a control for the
same firm's outcomes in a country that does have Dist==1 at some
point.  It's not clear to me whether you want to have firm FE and
country dummies interacted with Dist.  Perhaps this would all be clear
if you could be a bit more specific about the research design...

You may just want
 xtset id_var
 xtreg Yist Dist, fe
 xtset id_var
 tab country, gen(_c)
 drop _c1
 xtreg Yist Dist _c*, fe
but you need to be clear about what comparison you are trying to draw.

On Wed, Feb 18, 2009 at 8:21 AM, Fiorenzo Bovenzi
<> wrote:
> Dear list,
> I am trying to estimate a difference-in-difference model with panel
> data, i.e. Yist (where s indicates the treatment/control group) as a
> function of a dummy Dist equal to 1 for the treatment group after the
> treatment occurs, and 0 otherwise.
> The basic steps I follow to do this are:
> tsset id_var t_var
> xtreg Yist Dist, i(id_var) fe
> However, when I use tsset, I receive the message "repeated time values
> in sample" because units i are called the same in both the treatment
> and control groups - e.g. same firm in country A (treatment) and
> country B (control).
> Is there a way to fix this, other than assigning a different id to
> each unit in the treatment and control groups?  Many thanks,
> fiorenzo
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