[VoxBo] permutation tool question

[Lila Chrysikou]

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
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