[VoxBo] permutation tool question

[Lila Chrysikou]

Thanks, David.
I am suspecting it has something to do with linearity limitations, however,
I am unsure as to how these calculations are "translated" in voxbo and/or
how one determines what's good enough.
thanks again,
~L

On 10/22/07, David January <djanuary at sas.upenn.edu> wrote:
>
> I don't know the answers to your overall questions, Lila, but I will
> say that the number of possible permutations of N things does not grow
> linearly with N.  For example, there are 6 permutations of 3 things,
> but only 2 permutations of 2 things (you can probably find the formula
> for calculating the number of permutations on wikiepedia).
>
> -David
>
> On 10/22/07, Lila Chrysikou <evangelg at psych.upenn.edu> wrote:
> > Hello everyone,
> >
> > I was wondering if there is a specific number of subjects and/or data
> points
> > required for VoxBo to run a sufficient number of permutations.
> >
> > For contrasts including data from 12 subjects the permutation tool
> allows
> > for a minimum of 1000 permutations to be performed. However, for
> contrasts
> > including data from 6 subjects, vbpermgen's default permutation number
> > switches to 32 and the maximum number of permutations completed is 64.
> If I
> > try to change the number to 1000, I receive an error message that the
> data
> > in the particular contrast allow for a maximum of 32 permutations to be
> > performed (at times I also got a message for 0 permutations allowed,
> > depending on which computer of the cluster I was using).
> >
> > I should note that there is nothing different in the setup of the
> contrasts
> > relative to those including data from 12 subjects, for which the
> permutation
> > tool works without problems (hence I was expecting that with half data
> > points, I should be able to run at least 500 permutations).
> >
> > Is there a way to fix this problem or the data are not enough for the
> > process to work with accuracy? Or are the 32/64 permutations sufficient
> to
> > obtain a usable critical t?
> >
> > Many thanks for your help,
> >
> > ~Lila
> >
> > --
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Evangelia G. Chrysikou, Ph.D.
> > Post Doctoral Research Fellow
> > Thompson-Schill Lab
> >
> > Center for Cognitive Neuroscience
> > Department of Psychology, University of Pennsylvania
> > 3720 Walnut Street, Solomon Lab Room B51,
> > Philadelphia, PA 19104-6241
> >
> > Office address: 3810 Walnut St.,Room 307, Philadelphia, PA 19104
> > 215. 573.6726  (phone)
> > 215. 898.1982  (fax)
> > E-mail: evangelg at psych.upenn.edu
>
>
> --
> David January
> Department of Psychology
> University of Pennsylvania
>
>


-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Evangelia G. Chrysikou, Ph.D.
Post Doctoral Research Fellow
Thompson-Schill Lab

Center for Cognitive Neuroscience
Department of Psychology, University of Pennsylvania
3720 Walnut Street, Solomon Lab Room B51,
Philadelphia, PA 19104-6241

Office address: 3810 Walnut St.,Room 307, Philadelphia, PA 19104
215. 573.6726  (phone)
215. 898.1982  (fax)
E-mail: evangelg at psych.upenn.edu
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