Re: st: two-sample vs. two-mean

[Richard Goldstein <richgold@ix.netcom.com>]

ttest mpg1=mpg2 gives a paired t-test

after stacking, ttest mpg, by(group) gives an unpaired t-test

if you replace the first t-test by

ttest mpg1=mpg2, unp

you will match the result after stack

Rich

Martin Weiss wrote:
> Dear Statalisters,
>
>
> Try this in Stata:
>
> **********
> webuse fuel, clear
> qui{
> *two-sample mean comparison
> noi ttest mpg1==mpg2
> local pl1=r(p_l) 
> local pu1=r(p_u)
> local p1=r(p)
>
> *two-group mean comparison
> stack mpg1 mpg2, into(mpg)  clear
> rename _s group
> noi ttest mpg, by(group)
> local pl2=r(p_l) 
> local pu2=r(p_u)
> local p2=r(p)
>
> *Results
> noi di  _newline
> noi di in red "Results:" _newline
> noi di in red "Test against H1: <" _column(23)  round(`pl1', .001) " versus
> " round(`pl2', .001) _newline 
> noi di in red "Test against H1: >" _column(23)  round(`pu1', .001) " versus
> " round(`pu2', .001) _newline
> noi di in red "Test against H1: !=" _column(23) round(`p1', .001) " versus "
> round(`p2', .001) _newline
> }
> **********
>
> Then let me know how I can easily explain the difference between a
> "two-sample mean" and a "two-group mean" comparison test.
>
> Warm regards,
> Martin Weiss
> _________________________________________________________________
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>
>
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