Hello SPM-users,
having read the archived mails on FDR multiple test correction in VBM I am
still not sure about how to proceed with the preliminary results.
The study comprises high resolution T1WI of n=80 subjects, processing acc.
to a modified optimzed VBM protocol including Jacobian modulation and
smoothing at FWHM 12 mm. Six multiple correlationsmodels are set up, the
main regressor of interest being a behavioural result (psychological test
battery).
For simpl. I use the term 'activation' for voxels that are significant after
applying a directed t-test on the main regressor.
At uncorrected p < 0.001 correlation maps show activations well balanced
between focal/regional (not too excessive), also reasonable variation
between the tests is found. Correction at FDR 0.05 tends to lead to rather
excessive activations in most contrasts.
Using a more stringent q < 0.005 brings produces areas about comparable with
p uncorr 0.001. However, with the FDR being adaptive this differs between
contrasts: for rather weak results uncorrected 0.05 seems more appropriate.
Using teh script FDRill the T-distribution is clearly shifted (up to about
+2), so the question arises which inferences I can draw from such a model.
The questions are:
1. Is FDR appropriate and how to I deal with the adaptiveness of this type
of multiple test correction, i. e. is it appropriate to use different q
values to different contrasts - which obviously contain results at
'different strength'?
2. Can results - as earlier suggested - be presented i the way that
uncorrected p=0.001 clusters are listed and such clusters are reported which
show FDR lower than 0.05? (which would include the ones surviving lower
thresholds?)
3. T-distribution problem: I have not modelled total GM volume as covariate
because we were also looking for regional (e. g. a whole lobe) effects and
thought this would be overcorrected by such a COV? How can the conditions to
use FDR be achieved?
Any help is appreciated,
Thanks very much in advance!
Philipp Saemann
Max-Planck-Institute of Psychiatry
Kraepelinstr. 2-10
80804 Munich
Germany
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