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Hi Joe: A method for combining several individual Z-score maps to create a group statistical map is described in: Clark VP, Maisog JM, Haxby JV. fMRI Study of Face Perception and Memory Using Random Stimulus Sequences. J. Neurophysiology 79:3257-3265, 1998 The authors make use of fact that the sum of squared Z score values has been shown to have a chi-square distribution. For more information on that derivation see: Hugill, MJ. Advanced Statistics. London: Bell and Hyman, 1985. Tim Joseph Tracy wrote: > > Dear SPM Group: > > I have SPM z score volumes on 5 individuals, each z score volume > resulted from the identical contrast following the same experimental fMRI > run. Each individual's z score volume has also been rethresholded. I > want to create a group volume. > > Is it valid to just average across individuals and create a mean z score > volume? > > Other methods such as summing the z volumes and dividing by > square root of k have been suggested to me (k represents the number > of z score maps, which in this case would be 5). > > Any thoughts? > Thank you. Joe Tracy > > I want s on on five ind -- Timothy M. Ellmore Laboratory of Brain and Cognition National Institute of Mental Health Building 10, Rm 4C-104 10 Center Dr., Bethesda MD 20892-1366 phone: (301) 435-4939 fax: (301) 402-0921 email: [log in to unmask] http://hippo.nimh.nih.gov/people/tellmore/tellmore.html %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%