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
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