Not sure why your having the problem, but your design has several issues:
(1) There is no subject term;
(2) Randomise should not be used for designs with multiple factors due
to the fact that you can't properly permute the data.
As an alternative to your current design, you should form difference
images for each subject, then enter the paired differences into a
2-sample t-test. This would be a model that randomise could handle to
test group differences in the scan effect.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Wed, Oct 10, 2012 at 5:22 PM, Stephanie Groman <[log in to unmask]> wrote:
> Hello everyone:
>
> I have a bizarre output from a randomise analysis. I am hoping someone may
> have some insight into why this occurred.
> I collected structural MRI scans in monkeys before and after either
> receiving a drug or saline (essentially a 2x2 design) and am interested in
> the scan by group interaction.
>
> The matrix was setup as such (for simplicity, it's 4 subjects (N=2 per
> group)):
> Control Drug
> Sub1 Scan1 control 1 0
> Sub1 Scan2 control -1 0
> Sub2 Scan1 control 1 0
> Sub2 Scan2 control -1 0
> Sub3 Scan1 drug 0 1
> Sub3 Scan2 drug 0 -1
> Sub4 Scan1 drug 0 1
> Sub4 Scan2 drug 0 -1
>
> The contrasts used were:
> Control Drug
> Control: S1>S2 1 0
> Drug: S1>S2 0 1
> Control: S2>S1 -1 0
> Drug: S2>S1 0 -1
> Control < Drug -1 1
> Control > Drug 1 -1
>
> The TFCE and voxelwise outputs show significant clusters (or voxels) for the
> Control<Drug contrast; however, to my surprise, when I looked at the tstat
> output, the values were extremely small (<0.1). I am confused because I
> thought that the {output name}_tstat.nii files would contain values much
> greater than this (at least above the critical t value for this study
> t=1.64). Also, I'm not sure if this is a problem, but randomise failed to
> output the {output_name}_tfce_tstat files.
>
> Any help or insight would be greatly appreciated!
>
> Stephanie
>
>
>
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