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Re: st: margins and xtmixed
From
Ricardo Ovaldia <[email protected]>
To
[email protected]
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 <[email protected]> wrote:
> From: Ricardo Ovaldia <[email protected]>
> Subject: Re: st: margins and xtmixed
> To: [email protected]
> 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 <[email protected]>
> wrote:
>
> > From: Ricardo Ovaldia <[email protected]>
> > Subject: st: margins and xtmixed
> > To: "Statalist" <[email protected]>
> > 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
> >
> >
> >
> > *
> > * For searches and help try:
> > * http://www.stata.com/help.cgi?search
> > * http://www.stata.com/support/statalist/faq
> > * http://www.ats.ucla.edu/stat/stata/
> >
>
> *
> * For searches and help try:
> * http://www.stata.com/help.cgi?search
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
>
*
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