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# Re: st: question: an example of xtmixed: testing differences between groups at specific times

 From Joerg Luedicke To statalist@hsphsun2.harvard.edu Subject Re: st: question: an example of xtmixed: testing differences between groups at specific times Date Wed, 9 Jan 2013 09:20:03 -0500

```If by "compare" you mean hypothesis testing, try

margins girl, at(age=(0.5 1 1.5 2)) coefl post

Then, for example, testing the difference between boys and girls at
baseline would be:

test _b[1bn._at#0bn.girl] = _b[1bn._at#1.girl]

To test the difference between age 0.5 and age 1 for boys would be:

test _b[1bn._at#0bn.girl] = _b[2._at#0bn.girl]

If you were interested in estimating the difference including a
confidence interval between age 0.5 and age 1 for boys, you could
type:

lincom _b[2._at#0bn.girl] - _b[1bn._at#0bn.girl]

Joerg

On Tue, Jan 8, 2013 at 8:49 PM, Sun, Wensheng <wsun@bcm.edu> wrote:
> Hi,
>
> I am using –xtmixed- on a longitudinal study with planned measurements at 0, 6, 12 and 24 months, but very few follow-up measurements at the planned time. I have fitted the model and need to evaluate changes from baseline and also differences between two treatment groups at exactly 6, 12 and 24 months. The –xtmixed- followed by –margins- gives margins and standard error at each time. How do I compare them? I am using Stata 12 for Windows.
>
> An example of that in public shared data should be
>
> . webuse childweight
> . xtmixed weight c.age##i.girl || id: age, cov(uns) res(ind) mle variance
> . margin girl, at(age=(0.5 1 1.5 2))
>
> What STATA commands should I use if I want to compare the weight between boys and girls at age of 0.5, 1, 1.5, and 2? and How should I compare changes from .5-year to 1 or 1.5 or 2 in girls and boys?
>
> Thank you very much!
>
>
> Carol
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```