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Re: st: comparing different means using ttest


From   David Lempert <david.lempert@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: comparing different means using ttest
Date   Thu, 16 Dec 2010 15:48:05 +0100

Thanks a lot Maarten I will look into this!

On Thu, Dec 16, 2010 at 3:32 PM, Maarten buis <maartenbuis@yahoo.co.uk> wrote:
> --- 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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