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Any time that you have a covariate with a large range and that covariate is associated with your DV, it should be included. It will reduce the residual error of the model and increase the significance. The key question is whether the covariate has the same relationship with the DV in each group. If the DV/covariate relationship is not significantly different between the groups, then you only need a single column for each covariate. If the relationship is different, then you need to model each covariate as N columns for N groups. See mumford.fmripower.org/mean_centering/ for more details. In brief, mean centering across everyone leads to covariate-adjusted means (e.g. what are the group means if each group had the same covariate value); mean centering within each group leads to the same group means, but you are controlling for the covariates; not mean centering leads to the group terms being the y-intercepts. Hope this helps. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Postdoctoral Research Fellow, GRECC, Bedford VA 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 Tue, Nov 13, 2012 at 8:45 AM, Maria Serra <[log in to unmask]> wrote: > Dear SPM experts, > > we have performed a VBM analysis with 4 groups (3 with different grade of > pathology and 1 healthy control group). We carried out an ANOVA between 4 > groups with no covariates given that there were no significant differences > between groups regarding variables which could affect gray matter volume > (TIV, Age, Gender, Schooling..). > > Even though, reviewers have suggested us to include TIV and Age as > covariates, would it be correct? in case of including them, would they act > as a confounding agent, introducing noise to the analysis? > > Thank you in advance for the help. > > > -- > Maria Serra > > PIC (Port d'Informació Científica) > Campus UAB, Edifici D > E-08193 Bellaterra, Barcelona > Telf. +34 93 586 8232 > www.pic.es > > __ > > Please consider the environment before printing this e-mail > > >