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Hi - we would need to know more specifics about the models you used, to comment on this error message. Please see Jeanette Mumford's web page for advice on modelling and demeaning. Cheers. On 24 Oct 2011, at 14:08, Amandine Pelletier wrote: > Dear FSL experts, > > I'm working on a cohort of 154 subjects, two types of scans were acquired : anatomical MRI and DTI scans. > > I performed two TBSS analyses (with randomise=5000 permutations) : > 1/ First analysis: MD values measured in hippocampus (values were extracted to mask of segmented hippocampus created with FIRST tool) were adjusted for age, sex, level of education in a general linear model. > 2/ Second analysis: Volumes of hippocampus (in the same manner, the values were extracted to mask of segmented hippocampus created with FIRST tool) were adjusted age, sex, level of education and total intracranial volume (TIV) in a general linear model. > > However for the first analysis, when I created the design, I obtain the following error message: > "Problem with processing the model: Warning: at least one EV is (to close to) a linear combination of the others. You probably need to alter your design. (Design matrix is rank deficient - ratio of min:max eigenvalues in SVD of matric is 0)." Nevertheless, the GLM design is created. For the second analysis, I didn't obtain this following message. > Has knowledge that, Age, TIV, Volumes of hippocampus and MD values of hippocampus are demeaned. > > I don't understand this error message, Is it that MD values are too lower ? Can anyone help with interpreting the reason for this error message ? > Is it can have consequences on my results ? > > Moreover, I would like to understand why is it recommended to demean the continuous variables ? > > Any suggestions or tips would be very appreciated, > Thanks for your time and your help. > > Amandine > --------------------------------------------------------------------------- Stephen M. Smith, Professor of Biomedical Engineering Associate Director, Oxford University FMRIB Centre FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK +44 (0) 1865 222726 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~steve ---------------------------------------------------------------------------