Re: st: margins and xtmixed - revised

[Joerg Luedicke <joerg.luedicke@gmail.com>]

You should plot the trajectories for each of your treatment groups.
Then you can immediately see whether there are any differences across
treatment arms or not. Whether these differences will be statistically
significant will then simply be a matter of your sample size, study
set-up, quality of measurements, observed/not observed covariates etc.

However, if you really need a test, you want to have some kind of
hypothesis. Your test is relatively unspecific: it tells you whether
there is at least one difference between treatment arms at one given
point in time.

Anyway, I would say plot your trajectories and put confidence bands around them.

J.

On Fri, Mar 16, 2012 at 6:41 AM, Ricardo Ovaldia <ovaldia@yahoo.com> wrote:
> I am clarifying my prior query, because I messed up and perhaps confused folks with my prior post.
> Here is the study:
> Patients were randomly allocated to three treatment groups. Each patient was followed for a year and measurements made at baseline and every three months (month=0,3,6,9,12). Interest is on the effect of treatment overall, and at specify times (months) after controlling for sex and bmi. I used –xtmixed- allowing both a random intercept and random slope, and month by group interaction:
>
> .xtmixed ft i.group##month i.sex bmi|| id:month, cov(unstructured) mle
>
> To compare groups at given times, I used –margins, post- and –test-:
> . margins, over(group month) post
> . 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
> So my questions are (1) most importantly is this correct? (2) are the delta-method SE being use by –test-? and (2) Is there a better way to compare groups at fixed times?
>
>
> Thank you in advance,
> Ricardo
>
>
> Ricardo Ovaldia, MS
> Statistician
> Oklahoma City, OK
>
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