Hector-Gabriel wrote:
> Dear SPM'ers:
>
> Looking among some fMRI/SEM studies I realised that the stacked model
> approach has been used to compare a specific anatomical model under
> different experimental conditions but most of the time the authors
> don't
> say anything about the goodness of fit (p value for example) of the
> base
> model. My questions are:
>
> 1) Is it possible to use SEM to evaluate not only the significant
> difference across different conditions but also the goodness of fit
> of the
> model?
>
>
>
>
> 2) Is it valid to use, in a stacked model approach, an anatomical
> model
> that is not statistically significant (p value < 0.05 for example)?
>
1. The goodness of fit of the base model can be
assessed using a Chi^2 criterion. For details of the derivation of this
criterion
see Bollen (1989) and for a description of it in action in a
neuroimaging context
see Bullmore et al. (2000).
2. In my opinion the base model *should be* statistically significant.
But there
is, I believe, some disagreement among researchers on this issue. On the
second and third pages of the Bullmore paper, for example, they provide
reasons
why absolute (as opposed to relative) assessment of models should be
made with caution. Maybe others would like to comment.
Hope this helps,
Will.
References:
@Article{bullmore_sem,
author = {E. Bullmore and B. Horwitz and G. Honey and
M. Brammer and S. Williams and T. Sharma},
title = {How good is good enough in path analysis of fMRI
data ?},
journal = {Neuro{I}mage},
year = 2000,
volume = 11,
pages = {289-301}
}
@Book{bollen_sem,
author = {K.A. Bollen},
title = {Structural Equations with Latent Variables},
publisher = {John Wiley, New York},
year = 1989
}
--
William D. Penny
Wellcome Department of Cognitive Neurology
University College London
12 Queen Square
London WC1N 3BG
Tel: 020 7833 7478
FAX: 020 7813 1420
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URL: http://www.fil.ion.ucl.ac.uk/~wpenny/
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