On Mon, 17 Mar 2008 11:22:03 -0700 (PDT), "srbraithwaite@[EMAIL PROTECTED]
"
<srbraithwaite@[EMAIL PROTECTED]
> wrote:
> I'm attempting to fit growth curve models in AMOS for variables with
> three time points using condition (which is dichotomous) as an
> exogenous predictor. I believe I have everything set up so the model
> should work. I adjusted the ICPT and SLOPE so they are free to vary
> (since estimating means and intercepts in AMOS changes the mean of
> latent vars to 0). The loadings from ICPT to the 3 observed variables
> are all set to 1 and the loadings from SLOPE to the indicators are
> fixed at the number of weeks from baseline (0, 6 and 40). I adjusted
> the observed indicators so their intercept is 0 and their disturbance
> terms are constrained to be equal.
Just a couple of comments that suggest my ignorance
about the problem, but which may inspire someone else -
I don't know about the tool you are using, AMOS, but "three
time points" is pretty minimal for doing *any* growth
modeling.
This is a poor model/design in one regard - If (0, 6, 40) weeks
were reasonably expected to show equal-interval effects,
distances should be modeled as "equal" and not as the
number of weeks. For my curiosity -- What are you hoping
to get from this that you would not get from linear slopes
in ANOVA, for each of the several variables?
>
> Here's the problem. When I attempt to correlate the disturbance terms
> for ICPT and SLOPE, it says that the solution is inadmissible because
> the matrix between the disturbance terms is not positive definite.
> When I remove the correlation between the disturbance terms for ICPT
> and SLOPE, it says the solution is inadmissible because the variance
> (or disturbance term) for SLOPE is negative. This has happened for
> more than one of the variables I am attempting to model.
>
> What am I doing wrong?
--
Rich Ulrich
http://www.pitt.edu/~wpilib/index.html
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