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Re: setting up a one-way ANOVA with VBM data

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Fri, 14 Aug 2009 10:18:53 +0200

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 ```Hi - you'll need to do the 'anova maths' to check this I'm afraid: sometimes apparently equivalent models/contrasts are what they appear; other times, things can be counterintuitive. Cheers -------------------- Stephen M. Smith, Professor of Biomedical Engineering Associate Director, Oxford University FMRIB Centre FMRIB, JR Hospital, Headington, Oxford. OX3 9 DU, UK +44 (0) 1865 222726 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~steve ---------------------- On 13 Aug 2009, at 14:43, Colm Connolly <[log in to unmask]> wrote: > > On 4 Oct 2008, at 09:59, Thomas Nichols wrote: > >> Setting up a fourth variable would lead to a linearly-dependent >> design matrix. I.e. the sum of EV2, EV3 and (your proposed) EV4 >> would equal EV1. If you dropped the grand mean covariate, you >> could take the approach you suggest, and, in fact, that's the dummy >> variable approach taken inhttp://www.fmrib.ox.ac.uk/fsl/feat5/ >> detail.html#FTests . > > > Sorry to resurrect an old thread. > > I'm doing similar thing as Ben at the moment and am wondering why > you would choose the grand-mean approach to specifying the EVs and > contrasts versus the (simpler to my mind) approach of using the > dummy variable? > > If I under stand this correctly (assuming n=3 and there are 3 groups > and no repeated measures) > long mid short age > 1 0 0 12 > 1 0 0 11 > 1 0 0 13 > 0 1 0 11 > 0 1 0 17 > 0 1 0 14 > 0 0 1 10 > 0 0 1 12 > 0 0 1 16 > > and contrasts (* indicates inclusion in an F-test) > 1 0 0 0 * > 0 1 0 0 * > 0 0 1 0 * > 1 -1 0 0 #long > mid > 0 1 -1 0 #mid > short > > will allow me to ask the question "is there a group effect (the F- > test)" and then to tweeze apart some of the possible pair-wise > comparisons having accounted for variance attributable to age. Am I > correct? > > Regards, > -- > Dr Colm G. Connolly > Institute of Neuroscience > The Lloyd Building > University of Dublin > Trinity College, Dublin 2, Éire > Fax: +353-1-671-3183 > > Please note that electronic mail to, from or within the Trinity > College Dublin, may be the > subject of a request under the Freedom of Information Act. > > > ```