Dear SPM experts,
I´m sorry to ask about this topic again but I ´m really not sure whether I
do the correct analysis.
I want to compare the grey matter volume of 2 groups using total grey matter
volume as covariate of no interest. So, I´m using the full factorial model
where I define a singel factor (group) with 2 levels, independent, unequal
variance and ANCOVA=yes. Then I load the corresponding scans (2 cells) and
finally my covariate. When I run the job I see the following:
For AnCova by sF2, SPM needs estimates of global activity.
But you have specified to omit this computation.
SPM has overridden this omission and will automatically compute
globals as the mean value of within brain voxels
Does this mean that SPM is doing global correction and does not use my
vector of global values as covariate of no interest which I specified in the
Covariate section? I have read that in case of VBM and ANCOVA it´s better to
use the own global values and this is what I actually need. So, if I specify
ANCOVA=no under the Factor section and Global calculation=User, is this the
correct analysis?
I would be really grateful if anybody could help me with this and I hope
that I´m not totally wrong with my assumptions.
Thank you very much in advance!
Best regards,
Beate
|