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


From   "Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: How do I graph prediction of mean growth trajectory?
Date   Sat, 22 May 2010 18:35:04 -0700

Dear David

I think that it might help if we were able to see a picture of the graph that you have in mind. Of course, sharing such a picture is not easy via Statalist, but you could draw something and share it using Jing, see

http://www.michaelnormanmitchell.com/stow/capturing-and-sharing-screen-images.html

I think that the growth model that you have in mind does not involve any of the interaction effects (note that age and age squared are not interacted with anything) thus when you look at the trajectory over time, the interactions you refer to just get "averaged out". I think it is possible that you might actually want more than one graph, for example you might like...

1) a graph with age on the X axis, the predicted outcome on the y axis, and separate lines for maternal IQ groups/treatment groups. This shows the growth curve over time for the six groups. Note that the growth curves will be parallel.

2) a graph with maternal IQ class on the X axis, separate lines for treatment groups. This will focus on the interaction.

There may be multiple ways to touch the elephant, but I don't think there is one way to touch the elephant to get the entire picture. It is also possible that you might be interested in interacting age (and perhaps age squared) with some of your other predictors if you think that the growth trajectory depends on these factors.

Finally, I would recommend "Applied Longitudinal Data Analysis: Modeling Change and Event Occurrence" by Judith D. Singer and John B. Willett to anyone who is interested in these kinds of models. It does not solve this exact problem, but is an outstanding reference, see

http://www.ats.ucla.edu/stat/examples/alda.htm

I hope this helps,

Michael N. Mitchell
Data Management Using Stata      - http://www.stata.com/bookstore/dmus.html
A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html
Stata tidbit of the week         - http://www.MichaelNormanMitchell.com



On 2010-05-22 5.52 PM, David Torres wrote:
Hello,

I actually came upon the link Nick provided in a previous post
(http://www.ats.ucla.edu/stat/stata/faq/mar_graph/margins_graph.htm)
while I was working on this earlier today. The graph I was able to
produce does seem useful in that it shows the difference in slope
between child IQ scores of the different maternal IQ classes for both
the control and treatment groups. I wonder, though, if I'm missing
something by not showing growth. ???

Since the data include assessments at several time points (8 to be
exact), should I not want a graph of the adjusted means at each time by
maternal IQ and treatment group assignment? And shouldn't it be
curvilinear given the quadratic? I guess I would like a growth curve
that takes into account significant interaction effects.

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

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


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