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Yeah, thanks - very educative discussion! Have a nice holiday, Cornelius On Tue, Dec 21, 2010 at 2:20 PM, Jeanette Mumford <[log in to unmask]> wrote: > Thanks guys! > > Jeanette > > On Tue, Dec 21, 2010 at 3:31 AM, Stephen Smith <[log in to unmask]> wrote: >> >> One final point to add to the question of what FEAT does in practice: >> >> And now finally, to Jeanette's original question - I think (and Steve or >> others can correct me if I'm wrong) that the orthogonalisation in FEAT is >> done one by one, so that if you do x1 wrt x2 and x2 wrt x1, then it does the >> second after the first one has been done already, meaning that it uses the >> new EVs, not the original ones, and so these become orthogonal after the >> first step and nothing happens in the second step. It probably isn't quite >> what you'd want in this condition theoretically, but I honestly cannot ever >> see anyone needing such a set of orthogonalisations as in the examples >> given, so in that case I think it is not really a problem. >> >> That's right - FEAT applies each selected EV's orthogonalisation in series >> rather than all at the same time - which as MJ rightly says seems odd, but >> isn't a problem because it doesn't make sense to tell FEAT to 'do it both >> ways' anyway. >> However, a related question, in case anyone is concerned given this >> answer: If you tell one EV to become orthogonal to *more than one other >> EV*, then that case *is* handled correctly - it is orthogonalised to the >> full space spanned by the set of EVs selected, and not orthogonalised to >> each in series (which would *not* in general give the correct >> orthogonalisation). >> Cheers. >> >> >> >> >> On 12/20/2010 01:16 PM, Jeanette Mumford wrote: >> >> Hi! >> >> I'll start off by saying I think orthogonalization of regressors makes >> >> absolutely no sense in almost all cases, regardless, I'm curious as to >> >> how FEAT handles cases where people go a little wild and start >> >> orthogonalizing every ev with respect to all other evs. >> >> For example, my (rusty) geometry tells me that if I have two >> >> regressors, x1 and x2 and if I then orthogonalize each one with >> >> respect to the other to get x1_wrt_x2 and x2_wrt_x1, then cor(x1, x2)= >> >> -1* cor(x1_wrt_x2, x2_wrt_x1). When I ask FEAT to orthogonalize each >> >> of two regressors with respect to the other it actually didn't alter >> >> x2. Thus, the correlation in the orthogonalized FEAT model is 0. >> >> Further if somebody had 3 or more regressors and (wrongly) >> >> orthogonalized everything with respect to everything else, what does >> >> FEAT do? It seems with all the orthogonalization it would be very >> >> difficult to correctly interpret hypothesis tests. Is there a >> >> meaningful interpretation? >> >> Thanks! >> >> Jeanette >> >> >> --------------------------------------------------------------------------- >> 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 >> >> --------------------------------------------------------------------------- >> >> >> > > -- Dr. med. Cornelius J. Werner Department of Neurology RWTH Aachen University Pauwelsstr. 30 52074 Aachen Germany Institute of Neuroscience and Medicine MR Physics - INM4 Research Centre Juelich 52425 Juelich Germany ::: Please encrypt confidential data :::