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st: How do I graph prediction of mean growth trajectory?


From   David Torres <torresd@umich.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: How do I graph prediction of mean growth trajectory?
Date   Fri, 21 May 2010 17:27:07 -0400

Hello all,

I am trying to graph expected growth in IQ scores by maternal IQ class and treatment/control group assignment. Since there are three maternal IQ classes and two group assignments, I should have six lines or growth curves.

Now, my models were calculated using Stata's -xtmixed- command, so I've been following Rabe-Hesketh and Skrondal's text on multilevel modeling, 2nd edition, page 210-220. They give an example of how to graph growth, but the example does not include interactions nor does it allow for the slopes to change between some other time-varying covariate, which I have in my model. The outcome is test score, of course. The list of the predictor variables in the full model from which I would like to create my graphs follow:

1.  age
2.  age^2
3.  assessment given (two different assessments over several years, so
    this is entered as a time-varying covariate.  It would be great if I
    could get the slope to vary by assessment) - Stanford-Binet or Wechsler
4.  home environment - 0/1, less stimulating/more stimulating
5.  treatment - 0/1, control/treatment
6 & 7.  maternal IQ class - 1/2/3, IQ<=75/IQ between 76&90/IQ between 91&110
       1 is the reference category
8 & 9.  interaction between treatment and maternal IQ class

All of the variables in this model are significant, so I want to make sure that my graph accurately reflects that. Also, I understand that since interactions can be difficult to interpret sometimes a visual represention of the data are always good as an accompaniment.

A related question is this: If I have to produce these lines separately, creating six graphs, is there a way I can overlay them?

--------------------------------------------

David Diego Torres, MA(Sociology)
PhD Candidate in Sociology
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