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Re: st: comparing different means using ttest


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: comparing different means using ttest
Date   Thu, 16 Dec 2010 14:32:57 +0000 (GMT)

--- On Thu, 16/12/10, David Lempert wrote:
> reg gdp_ year
> predict res, r
> 
> sum res (I check and see if the mean is close to zero,
> which it is - hence homoscedastic as I see it)

No, the mean of the residuals is trivially equal to zero
(ignoring precision issues). There is no need to test this,
and the fact that you found this to be true tells you 
exactly nothing other than that Stata is working as it 
should. If you want to look at homoscedsticity I would just
look at the plot created by -rvfplot-, see: 
-help regress postestimation-
 
Hope this helps,
Maarten

--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

http://www.maartenbuis.nl
--------------------------


      

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