Dear Nicola,
Perhaps, as your 'activations' seem consistent with your study hypotheses
you would be justified in using a 'Small Volume Correction' approach with
volumes centred around your hypothesised regions. You could then
pursue the FWE, p<0.05 criterion using these smaller search volumes.
Best wishes,
Will.
Nicola Pannacciulli wrote:
> Dear Will and SPMers,
> Thank you very much for your fast and clear reply. I still have a
> practical question about the correction for multiple comparisons to be
> answered. In my VBM analysis comparing two groups of subjects (patients
> vs. controls) I have some areas of the brain which are significant at
> voxel-level FWE-corrected P-values ranging from 0.005 to 0.087 and whose
> location is highly consistent with my study hypotheses. Nevertheless,
> since some of them are close (0.087) but not below the FWE-corrected P-
> value, I am not allowed to state in my report that I am going to discuss
> only the results surviving the FWE-correction and I am forced to go for
> the fall-back of FDR-correction. If I do this, I get a bunch of areas
> throughout the brain, some of them having an FDR-corrected P-value less
> than 0.05 but a FWE-corrected P-value of 0.998 which, I guess, are
> definitely not worth being reported. But, if I decide to go for the FDR-
> correction, I think I am not allowed to choose which results are really
> significant and I need to report all of them even if the FWE-corrected P-
> value is "unbelievably" not significant. I hope I made myself clear enough
> about the problem I have been currently struggling with. Do you have any
> suggestions? Thank you very much and happy holidays to you and the whole
> list. Best regards,
> Nico
>
>
--
William D. Penny
Wellcome Department of Imaging Neuroscience
University College London
12 Queen Square
London WC1N 3BG
Tel: 020 7833 7475
FAX: 020 7813 1420
Email: [log in to unmask]
URL: http://www.fil.ion.ucl.ac.uk/~wpenny/
--
William D. Penny
Wellcome Department of Imaging Neuroscience
University College London
12 Queen Square
London WC1N 3BG
Tel: 020 7833 7475
FAX: 020 7813 1420
Email: [log in to unmask]
URL: http://www.fil.ion.ucl.ac.uk/~wpenny/
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