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Dear Hans, > Thanks, but I am interested in a more detailed answer about exactly > how the residuals are permuted :). Here's my question again for > reference: > > --- > I have a question regarding randomise residuals. As per my > understanding randomise fits the null model only to the data and > calculates null only residuals. Then it permutes these null only > residuals and adds them back onto the fitted null model to create > realizations of null data. My question is: > > Are these null only residuals modified (or standardized) in any way > before permuting them? If so, exactly how? I'm no expert on randomise, but is seems pretty straightforward to me. Let's say you have a model with two groups and age as a covariate, and that your contrast happens to be [1 0], i.e. you are interested in effects of group after affects of age have been removed. By virtue of you contrast not spanning the age regressor randomise can identify it as a "confound" and regress out all effects of age. What is left is the residuals, i.e. that which in our model can be explained either by group or not at all. randomise will the permute these residuals (equivalent to permuting the group indicators), for each permutation fitting the GLM to all voxels and calculating the t- statistic. Depending on your inference it may then save away maximum voxel, maximum cluster size etc, thus building an empirical distribution of that statistic. I hope this is clear? Good Luck Jesper > --- > > Thanks, > Hans. > > > > On Thu, Jul 2, 2009 at 10:48 AM, Matthew Webster <[log in to unmask] > > wrote: > Hello Hans, > Randomise separates the input model into tested > and nuisance effects, the input data is adjusted for the nuisance > effects and this adjusted data is then fitted to the full permuted > model.. > > Many Regards > > Matthew > > Hi FSL experts, > > I have a question regarding randomise residuals. As per my > understanding randomise fits the null model only to the data and > calculates null only residuals. Then it permutes these null only > residuals and adds them back onto the fitted null model to create > realizations of null data. My question is: > > Are these null only residuals modified (or standardized) in any way > before permuting them? If so, exactly how? > > Thanks, > > Hans Tissot. >