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Re: st: Fwd: help with xtmixed and margins


From   James prince <[email protected]>
To   [email protected]
Subject   Re: st: Fwd: help with xtmixed and margins
Date   Tue, 25 Sep 2012 05:36:47 -0500

Paul,thank you for your suggestions. Regarding the t-test, I did
notice that MARGNS outputs SE. I converted them to SD (i.e.
sqrt(N)*se) and use the output in TTESTI. Notice in the output from
the t-test that the SE are the same as those reported by MARGIN,
however the p-values are very different from that reported by the TEST
command after MARGINS, POST. What am I missing?

Thank you again,
Jim

On Mon, Sep 24, 2012 at 1:44 PM, Visintainer, Paul
<[email protected]> wrote:
> Jim,
>
> The first thing to note is that, in your t-test command, you are using standard errors rather than standard deviations.  Because the -margins- command is based on the prior estimation command, it outputs standard errors.
>
> Secondly, -margins- is a powerful, but complex command.  I strongly recommend spending a lot time with the help file and its examples. (My printed copy is well-worn.)  Also, -margins- has options to run the types of tests you want to do -- either with options or with related commands: e.g., -contrast- and -pwcompare-.
>
> I would also recommend getting some of the books on Stata programming (e.g., Sophie Rabe-Hesketh series on multilevel models and Michael Mitchell's book on visualizing regression models are very helpful.  There are several other great texts at the Stata bookstore).  These have a multitude of useful examples and commands.
>
> Paul
>
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf Of James prince
> Sent: Monday, September 24, 2012 1:10 PM
> To: [email protected]
> Subject: Re: st: Fwd: help with xtmixed and margins
>
> 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 <[email protected]> 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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