 |
 |
When you include covariates, make sure you fully described how they were centered (and whether you allowed the relationship between your DV and covariate to vary by group) and how that effects the interpretation of the group differences; then I see no problem. You just need to be clear as to what is being tested. For example, if you have a covariate that is different in each group and you center based on the overall mean, then you want to state you are testing the difference in the covariate-adjusted group means, rather than the actual difference in group means. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Postdoctoral Research Fellow, GRECC, Bedford VA Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Website: http://www.martinos.org/~mclaren Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Thu, Feb 23, 2012 at 2:29 PM, Jonathan Peelle <[log in to unmask]> wrote: > Dear Pedro, > > > 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. > NeuroImage 60:1503-1516. > http://dx.doi.org/10.1016/j.neuroimage.2011.12.086 > > > Hope this helps! > > Best regards, > > Jonathan > > -- > Dr. Jonathan Peelle > Department of Neurology > University of Pennsylvania > 3 West Gates > 3400 Spruce Street > Philadelphia, PA 19104 > USA > http://jonathanpeelle.net/ >