Dear Haoran,
I am certainly not an expert user for SPM-DCM. Therefore, I cannot say
why the BMA fails in your case. On the other hand, I am happy to discuss
my previous comment in more detail.
For a fair comparison between models one should make sure that all
models were given sufficient chances to explain the data. If a model
does not reach convergence within 64 iterations it could be that the
model is inappropriate for the given data and it will never reach
convergence. Alternatively, it also could be that convergence is reached
later because e.g. the optimization started out from a choice of
parameters which was too far from the optimum and the fitting algorithm
needs longer to find the optimum. Ultimately, a small RV value shows
that your model explains most of the data.
Conducting BMS on models which may not have reached convergence leaves
you with the risk to evaluate these models below their potential value
for your data. This is shaky ground for your further discussion as you
mix up numerical problems with your attempts to identify the best
network description for your research question. Concerning the BMA, if I
understand it correctly, is relying on the best models only - so if your
non-converging models all have rather low F-values, they do not enter
the model averaging procedure.
all the best,
Burkhard
·ÉÄñ wrote:
> Dear Burkhard,
> ¡¡¡¡Would you please tell me the relationship between convergence and
> BMA? I confronted some problems and can't do BMA successfully. Really
> thank you very much£¡
> ¡¡¡¡Best regards!
> --
> Haoran
>
> At 2011-12-13 16:34:50,"Burkhard Maess" <[log in to unmask]> wrote:
> >Dear Robert and Haoran,
> >
> >I would rather disagree - a model which has not reached convergence
> >should not enter further analysis. With some luck the free energy value
> >is low and the model is ignored by BMA, but I would not bet on this.
> >
> >all the best,
> >Burkhard
> >
> >
> >·ÉÄñ wrote:
> >> Dear Robert,
> >> ¡¡¡¡Sometimes it will not converge within 64 iterations( or as you set
> >> 128 iterations), and I think you can use Ep.A B C for further analysis.
> >> ¡¡¡¡Best regards.
> >> --
> >> Haoran LI (MS)
> >>
> >> At 2011-12-13 03:30:51,"Robert Schulz" <[log in to unmask]> wrote:
> >> >Dear DCMers,
> >> >
> >> >already posted this question some time ago, I haven't received an answer yet. So please excuse if I ask again: In motor system DCMs with 6 nodes and grip left / right as modulatory influence, I don't get "convergence" after 128 iterations. What happened here. Is Ep. A B C though correct for further analysis?
> >> >
> >> >Thank you a lot,
> >> >Kind regards to you
> >> >
> >> >Robert
>
> >------------------------------------------------------------
> >Dr. Burkhard Maess
> >Max Planck Institute for Human Cognitive and Brain Sciences
> >Stephanstr. 1a, P.O. Box 500355, D-04303 Leipzig
> >Aussenstelle Bennewitz, phone/fax: +49(3425)887525-26/-11
> >mail: maess 'at' cbs.mpg.de, http://www.cbs.mpg.de
> >------------------------------------------------------------
> >Please do not attach files which may contain executable code,
> >e.g. .rtf is preferred over .doc(x)
>
>
>
--
------------------------------------------------------------
Dr. Burkhard Maess
Max Planck Institute for Human Cognitive and Brain Sciences
Stephanstr. 1a, P.O. Box 500355, D-04303 Leipzig
Aussenstelle Bennewitz, phone/fax: +49(3425)887525-26/-11
mail: maess 'at' cbs.mpg.de, http://www.cbs.mpg.de
------------------------------------------------------------
Please do not attach files which may contain executable code,
e.g. .rtf is preferred over .doc(x)
|