> I am currently running a series of structural MRI analysis using a large sample of subjects with psychosis, and I found myself in trouble.
> Do you think it is problematic to include several (three or four) nuisances at a two-group / full factorial analysis in SPM8?
I don't think it is necessarily a problem to include multiple
covariates in an analysis; however, there are two issues probably
worth keeping in mind. One is that if these factors differ across
groups, including them as covariates can be problematic. See e.g.:
Miller GA, Chapman JP (2001) Misunderstanding analysis of covariance.
Journal of Abnormal Psychology 110:40-48.
The second is that the way in which global gray matter effects are
accounted for influences how the results can be interpreted. We have
a recent paper that addresses this issue in the context of normal
aging, but it's applicable in other situations as well.
Peelle JE, Cusack R, Henson RNA (2012) Adjusting for global effects in
voxel-based morphometry: Gray matter decline in normal aging.
Hope this helps!
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104