Re: st: Fwd: help with xtmixed and margins

[James prince <jsprince26@gmail.com>]

I did not get a reply on my post from last week. So let me ask a
simpler question.

After a simple XTMIXED (intercept model and no interaction terms),
what is the difference between:

margins overweight, over( month)

and

margins, over( month overweight)

They yield different results.

Thank you,

Jim

On Fri, Sep 21, 2012 at 8:32 AM, James prince <jsprince26@gmail.com> wrote:
> Hi,
>
> I am new to Stata XTMIXED command and having some trouble. I hope
> someone could help.
> I am using the command MARGINS to predict the fitted means for what I
> thought was a simple mixed model (random intercept only).
> I have longitudinal data on children collected a 3, 6, 9, 12 and 24 months.
> I’m interested in the relationship between a physical activity score
> (score) and weight (dichotomized as overweight, yes, no).
> I’m treating “month” as discrete, and will like to test if the mean
> fitted score differs between by overweight status within month.
> Here is what I did:
>
> . noi xtmixed score i.overweight i.race i.month  bwt bln i.nedu
> i.gender || id:,mle
>
> <...>
>
>   I then use MARGINS with POST:
> . margins overweight,over( month) post
>
> Predictive margins                                Number of obs   =       1748
>
> Expression   : Linear prediction, fixed portion, predict()
> over         : month
>
> ----------------------------------------------------------------------------------
>                  |            Delta-method
>                  |     Margin   Std. Err.      z    P>|z|     [95%
> Conf. Interval]
> -----------------+----------------------------------------------------------------
> month#overweight |
>             3 0  |   100.9859   .4955637   203.78   0.000     100.0146
>    101.9572
>             3 1  |   99.36709   .6820952   145.68   0.000     98.03021
>    100.704
>             6 0  |   103.8597   .5121105   202.81   0.000     102.8559
>     104.8634
>             6 1  |   102.2409   .6479719   157.79   0.000     100.9709
>   103.5109
>             9 0  |   97.75468   .5267714   185.57   0.000     96.72223
>   98.78714
>             9 1  |   96.13588    .636223   151.10   0.000     94.88891
>   97.38285
>            12 0  |   102.2707   .5738975   178.20   0.000     101.1458
>   103.3955
>            12 1  |   100.6519   .6700738   150.21   0.000     99.33853
>   101.9652
> ----------------------------------------------------------------------------------
>
> And then TEST to test difference at each month. For example at 3 months:
> . test 3.month#0.overweight= 3.month#1.overweight
>
>  ( 1)  3bn.month#0bn.overweight - 3bn.month#1.overweight = 0
>
>            chi2(  1) =    7.05
>          Prob > chi2 =    0.0079
>
> However, when I do a t-test using the values form margin, a very
> different answer is observed:
>
> . ttesti 373 100.9859 9.5709248 96 99.36709 6.6831408
>
> Two-sample t test with equal variances
> ------------------------------------------------------------------------------
>          |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf. Interval]
> ---------+--------------------------------------------------------------------
>        x |     373    100.9859    .4955637    9.570925    100.0114    101.9604
>        y |      96    99.36709    .6820952    6.683141    98.01296    100.7212
> ---------+--------------------------------------------------------------------
> combined |     469    100.6545    .4189191    9.072283    99.83135    101.4777
> ---------+--------------------------------------------------------------------
>     diff |             1.61881    1.036685                -.418334    3.655954
> ------------------------------------------------------------------------------
>     diff = mean(x) - mean(y)                                      t =   1.5615
> Ho: diff = 0                                     degrees of freedom =      467
>
>     Ha: diff < 0                 Ha: diff != 0                 Ha: diff > 0
>  Pr(T < t) = 0.9405         Pr(|T| > |t|) = 0.1191          Pr(T > t) = 0.0595
>
>
> So, I have two questions:
> 1. How can I compare the adjusted scores by overweight status for each month?
> 2. What is the difference between these two MARGIN commands? They
> yield different results:
>
> . margins overweight, over( month)
> and
> . margins, over( month overweight)
>
> Thank you,
> James.
>
> *
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*
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