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Re: st: margins and xtmixed


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: margins and xtmixed
Date   Wed, 14 Mar 2012 12:45:34 -0700 (PDT)

I am sorry, I just realized my mistake in the model statement that I posted. The model that I actualy used was:

 xtmixed ft i.group##month || id: month, cov(unstructured) mle

I hope that clarifies things.

Ricardo
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


--- On Wed, 3/14/12, Ricardo Ovaldia <ovaldia@yahoo.com> wrote:

> From: Ricardo Ovaldia <ovaldia@yahoo.com>
> Subject: Re: st: margins and xtmixed
> To: statalist@hsphsun2.harvard.edu
> Date: Wednesday, March 14, 2012, 12:35 PM
> David Hoaglin asked:
> 
> >I am not sure that I understand the random-effects part
> of your model.
> >For each id it specifies a random intercept.  Also,
> for each id it
> >specifies a random coefficient for group.  Please
> say more about the
> >structure of your data.  How is id related to group
> (and to time)?
> 
> Patients were randomly allocated to three treatment groups.
> Each patient was followed for a year and measurements made
> at baseline and every three months. Interest is on the
> effect of treatment. The model allows for a random intercept
> and I also allow for each treatment to have a different
> slope (i.e. the -||Id:group-  part of the syntax
> statement). I am also allowing for and interaction between
> time (month and group). 
> 
> Ricardo
> 
> 
> --- On Wed, 3/14/12, Ricardo Ovaldia <ovaldia@yahoo.com>
> wrote:
> 
> > From: Ricardo Ovaldia <ovaldia@yahoo.com>
> > Subject: st: margins and xtmixed
> > To: "Statalist" <statalist@hsphsun2.harvard.edu>
> > Date: Wednesday, March 14, 2012, 8:24 AM
> > 
> > I am using –margins- and –test- after –xtmixed-
> to
> > test if “groups” differ at specific times. Time is
> > discrete in this study.
> > I am concerned with the small SE that margin reports
> > (delta-method adjusted). Small differences are found
> > significant.Any help would be appreciated
> > 
> > Here is an example:
> > 
> > I fit the following -xtmixed- model:
> > . xtmixed ft i.group##month || id: group,
> cov(unstructured)
> > mle
> > 
> > And I am interested in comparing the group effect at
> certain
> > time, 3 months for example, I used –margins, post-
> and
> > then –test-:
> > 
> > . margins, over(group month) post
> > 
> > Predictive margins           
> >                
> >     Number of obs   =   
> >     344
> > 
> > Expression   : Linear prediction, fixed
> > portion, predict()
> > over         : group month
> > 
> > 
> >
> ------------------------------------------------------------------------------
> >              | 
> >           Delta-method
> >              | 
> >    Margin   Std. Err. 
> >     z    P>|z| 
> >    [95% Conf. Interval]
> >
> -------------+----------------------------------------------------------------
> > group#month |
> >        1  3 
> > |   6.382194   .0760204 
> >   83.95   0.000 
> >    6.233196    6.531191
> >        1  6 
> > |   7.693377   .0819562 
> >   93.87   0.000 
> >    7.532746    7.854008
> >        1  9 
> > |   8.709543   .1006514 
> >   86.53   0.000     
> > 8.51227    8.906816
> >        1 12 
> > |   9.167792   .1323406 
> >   69.27   0.000 
> >    8.908409    9.427175
> >        2  3 
> > |   6.053284   .0700887 
> >   86.37   0.000 
> >    5.915913    6.190655
> >        2  6 
> > |   7.900573   .0777758   101.58   0.000 
> >    7.748135    8.053011
> >        2  9 
> > |   9.124052   .1023024 
> >   89.19   0.000 
> >    8.923543    9.324561
> >        2 12 
> > |   10.04959   .1374368 
> >   73.12   0.000 
> >    9.780223    10.31897
> >        3  3 
> > |   6.074073   .0780239 
> >   77.85   0.000 
> >    5.921149    6.226997
> >        3  6 
> > |   7.949809   .0856802 
> >   92.78   0.000 
> >    7.781879    8.117739
> >        3  9 
> > |   9.337498   .1128377 
> >   82.75   0.000     
> > 9.11634    9.558655
> >        3 12 
> > |   10.14703   .1480792 
> >   68.52   0.000 
> >    9.856798    10.43726
> >
> ------------------------------------------------------------------------------
> > 
> > . test 1.group#3.month= 2.group#3.month=
> 3.group#3.month
> > 
> > ( 1)  1bn.group#3bn.month - 2.group#3bn.month = 0
> > ( 2)  1bn.group#3bn.month - 3.group#3bn.month = 0
> > 
> >            chi2(  2)
> > =   12.22
> >          Prob > chi2
> > =    0.0022
> > 
> > Is this correct?
> > 
> > Thank you,
> > Ricardo
> > 
> > Ricardo Ovaldia, MS
> > Statistician 
> > Oklahoma City, OK
> > 
> > 
> > 
> > *
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> > *   http://www.stata.com/support/statalist/faq
> > *   http://www.ats.ucla.edu/stat/stata/
> > 
> 
> *
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> *   http://www.ats.ucla.edu/stat/stata/
> 

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