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Dear Tali, I just realised that I completely 'flipped' my advice on when variance smoothing is needed. I meant to say: Variance smoothing isn't really needed for *more than 20 DF*; you have exactly 20 here and I won't think it would do much. (When used, a variance smoothing equal to the original smoothing kernel is usually recommended). Thanks to Jesper Andersson for pointing this out! -Tom On Tue, Jun 25, 2019 at 7:00 AM Thomas Nichols <[log in to unmask]> wrote: > Dear Tali, > > I'm CCing this to the FSL list as this will be of interest to other users. > >> I like to compare 2 subjects (case study) to a group of 20 subjects. Can I use "-1" in randomise (sign-flipping)? >> >> No, that won't be the correct option. The -1 option is only for a > one-sample t-test, and, for VBM data especially, never applicable. > > For this design, you have 22-choose-2 or 231 possible permutations; all > p-values will be multiples of the smallest possible p-value 1/231=0.0043. > This seems like reasonable enough to conduct a test. > >> Should I add variance smoothing (-v) in addition to smoothing ? How can I estimate the appropriate value of -v? >> >> Variance smoothing isn't really needed for fewer than 20 DF; you have > exactly 20 here and I won't think it would do much. (When used, a variance > smoothing equal to the original smoothing kernel is usually recommended). > > -Tom > > -- > __________________________________________________________ > Thomas Nichols, PhD > Professor of Neuroimaging Statistics > Nuffield Department of Population Health | University of Oxford > Big Data Institute | Li Ka Shing Centre for Health Information and > Discovery > Old Road Campus | Headington | Oxford | OX3 7LF | United Kingdom > T: +44 1865 743590 | E: [log in to unmask] > W: http://nisox.org | http://www.bdi.ox.ac.uk > -- __________________________________________________________ Thomas Nichols, PhD Professor of Neuroimaging Statistics Nuffield Department of Population Health | University of Oxford Big Data Institute | Li Ka Shing Centre for Health Information and Discovery Old Road Campus | Headington | Oxford | OX3 7LF | United Kingdom T: +44 1865 743590 | E: [log in to unmask] W: http://nisox.org | http://www.bdi.ox.ac.uk ######################################################################## To unsubscribe from the FSL list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=FSL&A=1