[VoxBo] permutation tool question

[David January]

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