Dear SPM users,
power analysis of a single voxel (univariate power) can be easily applied
using the well known formulas. However the case of detecting an effect in
more than one voxel (mass univariate power) is different and I have found
only a few papers regarding this issue (Friston et al., NI 1996; Zarahn &
Slifstein, NI 2001; Desmond & Glover, J Neurosci M 2001). After reading the
papers I am still unsure about the right way to calculate power for VBM
data, which should be equivalent to a second level analysis of fMRI data.
The most appropriate way seems to be the approach of Zarahn & Slifstein
(Eq. 22). Is this the right formula to correct the power calculation for
mass univariate data? Has anybody tried this approach?
I guess this issue might be quite important for many people to estimate the
sample size needed to detect effects with effect size d using alpha level p
for mass univariate data with a given smoothness (or size of resels).
Best regards,
Christian
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____________________________________________________________________________
Christian Gaser, Ph.D.
Department of Psychiatry
Friedrich-Schiller-University of Jena
Philosophenweg 3, D-07743 Jena, Germany
Tel: ++49-3641-935805 Fax: ++49-3641-935280
e-mail: [log in to unmask]
http://dbm.neuro.uni-jena.de
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