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Re: st: Latent Growth Curve SEM GUI


From   Carter Rees <carterrees@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Latent Growth Curve SEM GUI
Date   Sat, 16 Jun 2012 00:52:56 -0500

Thank you Vince. Exactly what I was looking for. Just trying to get a feel for how the GUI works and the code. Hoping to be able to use Stata's SEM capabilities instead of switching to a new program.

Appreciate the help also Billy.

Carter 



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Carter Rees, Ph.D.
Assistant Professor
Arizona State University
School of Criminology and Criminal Justice
602-496-0495
carter.rees@asu.edu (mailto:carter.rees@asu.edu)



On Friday, June 15, 2012 at 11:21 PM , Vince Wiggins, StataCorp wrote:

> Carter Rees <carterrees@gmail.com (mailto:carterrees@gmail.com)> writes
> 
> > I have the basic latent growth curve model with 4 time points set up
> > using the SEM GUI. I would like to add one time invariant covariate
> > (gender) to predict the growth factors (intercept and slope) using
> > the GUI. However, each time I try to add this variable either as
> > latent or observed I get the error message "Cannot add path. Target
> > variable will become endogenous and is correlated with exogenous
> > variables". How do I add this via the GUI?
> > 
> > My goal is to replicate the results of an example from MPlus so I
> > can get a better feel for what the Stata code will look like and how
> > the GUI works. No troubles fitting the LGC without the covariate but
> > the covariate is stumping me.
> 
> 
> 
> I fear my response will not make much sense to anyone who does not look
> at the SEM diagram Carter refers to in a later posting in the thread,
> 
> > Slide 86 is what I am trying to do (not the same data but the model
> > spec is the same). But, I only want to use the observed variable
> > female. Syntax is on slide 87.
> > 
> > http://www.statmodel.com/download/Topic3-v.pdf
> 
> I suspect what Carter has done is create the top of the diagram first.
> That is he has created a measurement model with 4 measurements
> (math7-math10) and with two latent factors (i and s) <step 1>. I
> believe Carter then correlated the two latent variables (i and s) by
> adding a covariance path between the variables <step 2>. Then Carter
> added an observed variable (female) that he considers a determinant of
> i and s <step 3>. He then drew a path from female to i (or s) <step 4>. 
> At this last step a warning box notified him that,
> 
> "Cannot add path. Target variable will become endogenous
> and is correlated with exogenous variables."
> 
> You cannot model the correlation between an endogenous variable and any
> other variable. Such correlations are a side-effect of the model, not
> something that can be directly modeled. By adding a path pointing to
> i, Carter made that variable endogenous, but in step 2 he had
> previously correlated i and s. The SEM Builder cannot know what Carter
> intends. Does he want the correlation deleted, does he want it moved
> to the error for i?
> 
> The important thing to know is that the model Carter is specifying from
> the diagram does not correlate i and s. It correlates the errors from
> i and s. Carter never desired to correlate the variables themselves.
> 
> If I am guessing right, what Carter needs to do is delay step 2 until
> after he has drawn the paths from female to i and s. Then, he can add
> a covariance path between the errors for i and s.
> 
> 
> 
> -- Vince 
> vwiggins@stata.com (mailto:vwiggins@stata.com)
> 
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