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From |
James prince <jsprince26@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Fwd: help with xtmixed and margins |

Date |
Mon, 24 Sep 2012 12:10:15 -0500 |

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. > > * > * 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/

**Follow-Ups**:**RE: st: Fwd: help with xtmixed and margins***From:*"Visintainer, Paul" <Paul.Visintainer@baystatehealth.org>

**References**:**st: Fwd: help with xtmixed and margins***From:*James prince <jsprince26@gmail.com>

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