On Tue, 8 Feb 2005 16:48:30 +0000, Hyo Jong Lee <[log in to unmask]> wrote:
>Dear Group
>
>I have a basic question about global scaling and grand mean.
>I searched archives and found similar questions asked without
>clear answers to them.
>
>Should I set 'Global scaling' option during the first level
>analysis, if I plan to apply results to second level analysis
>for group comparison later?
A few years ago it was common for people to do global scaling in SPM fMRI
analyses. Nowadays the advice is that, in most situations, it shouldn't
be done. There have been extensive discussions of that on this mailing
list.
>The scale of baseline from each subject are not same. Thus,
>I am confused whether the first level analyses should have
>the global scaling option or spm2 handles this matter by default.
SPM does grand mean scaling implicitly (see further comments below).
>Also, what is purpose of setting 'grand mean' to a specific value?
The purpose of grand mean scaling is to attempt to deal with scaling
differences, without doing something as strong as global scaling.
In grand mean scaling, the mean over all (intracerebral, essentially)
voxels in all volumes in the session is computed. Then all voxels in all
volumes in that session are scaled by this mean (and perhaps then
multiplied by something like 100).
This takes care of scaling differences between *sessions*. It doesn't
address the issue of scaling differences between volumes within a
session. The presumption is that those latter differences aren't that
high. (Again, see the debate about whether global scaling shouuld be done
in fMRI.)
>Thanks in advance,
>Hyo Jong
|