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Re: st: is there a -regress- command equivalent to the pair matched ttest?


From   David Airey <david.airey@Vanderbilt.Edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: is there a -regress- command equivalent to the pair matched ttest?
Date   Tue, 9 Dec 2008 09:20:58 -0600

For the use of xtmixed for classical experimental designs, see this page:

http://www.ats.ucla.edu/stat/stata/faq/xtmixed.htm

-Dave

On Dec 9, 2008, at 9:12 AM, Sam Norton wrote:

Jose Maria

You can, if you really want to, use -xtmixed-.  Phil Enders
presentation at the West Coast Stata Users Group meeting in 2007 gives
an example of how to use -xtmixed- to estimate a RM-ANOVA.  Since
ANOVA is based on deviations from the grand mean -xi3- (available from
SSC) is needed to create effect coded variables.
http://www.stata.com/meeting/wcsug07/repeated_anova.pdf


--
Sam Norton
samjnorton@gmail.com
Research Student
Centre for Lifespan & Chronic Illness Research
University of Hertfordshire

2008/12/9 David Airey <david.airey@vanderbilt.edu>

A paired ttest is equivalent to a repeated measures ANOVA with two levels to the repeated factor. You can see the regression equivalent below.

webuse fuel, clear
list, clean
ttest mpg1 == mpg2
local t = r(t)
generate subject = _n
list, clean
reshape long mpg, i(subject) j(repeat)
list, clean
anova mpg repeat subject
anova, regress
test repeat
local F = r(F)
display `F' "=" `t'^2
xi: regress mpg i.repeat i.subject



On Dec 8, 2008, at 7:05 PM, josemaria wrote:

Dear Statalisters:
I am not sure whether it makes sense or not, but is there a sort of -regress- command that allows to conduct a pair-matched - ttest-, but with other independent variables?
I tried using -xtmixed-, but the result is not the same.
A contrived data set could be:
matching_identification  outcome ocasion variable_not_matched
                    1            1                1                0
                    1            2                2                0
                    2            3                1                0
                    2            5                2                1
                    3            2                1                0
                    3            4                2                1
                    4            6                1                1
                    4            5                2                1
To perform the pair-matched ttest it is necessary to -reshape- and to use -ttest outcome1==outcome2-, losing the independent variable_not_matched.
Thank for any orientation.
Regards,
José Maria

Jose Maria Pacheco de Souza, Professor Titular (aposentado)
Departamento de Epidemiologia/Faculdade de Saude Publica, USP
Av. Dr. Arnaldo, 715
01246-904  -  S. Paulo/SP - Brasil
fones (11)3061-7747; (11)3768-8612
fax (11)3714-2403
www.fsp.usp.br/~jmpsouza
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