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# Re: st: Difference-in-Difference with N=4, missing std. errors

 From Steven Samuels To statalist@hsphsun2.harvard.edu Subject Re: st: Difference-in-Difference with N=4, missing std. errors Date Sun, 5 Dec 2010 14:05:36 -0500

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I should add: Any conclusion of the three d.f. analysis might be wrong, because the within-country design and standard errors are ignored.
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Steve
sjsamuels@gmail.com

On Dec 5, 2010, at 1:40 PM, Steven Samuels wrote:

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A make-shift solution is to enter the original eight observations into -reg- with an added country indicator. This will give standard errors with three degrees of freedom.
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On Dec 5, 2010, at 11:37 AM, Stas Kolenikov wrote:

That's a theory of linear models issue: you have 4 equations with 4
unknowns, so you consume all information in the sample; you don't have
any degrees of freedom to estimate variance of the error term in the
regression, and hence end up with no standard errors or F-tests.

On Sun, Dec 5, 2010 at 10:10 AM, Jennifer Maki
<jenanneharris@hotmail.com> wrote:
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Statalist:I'm trying to use difference in differences to tease out the effect of a policy change. I'm using country level data and have yearly figures for both countries. I averaged them so that I have a figure representing the 5-year average pre & post treatment for both countries. leaving me with 4 observations. When I estimate this using the regress command in Stata, the coefficient on the interaction term is what I get when I calculate the difference in difference by hand, but I don't get standard errors or p-values in the output. Is there a way to get this information so that I can present the statistical significance of my findings?
```Thanks you,Jen
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Stas Kolenikov, also found at http://stas.kolenikov.name
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```