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

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Fri, 21 Sep 2012 08:32:01 -0500 |

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/

**Follow-Ups**:**Re: st: Fwd: help with xtmixed and margins***From:*James prince <jsprince26@gmail.com>

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