No, not at all sure. That is why I am asking. However, mri_glmfit says that the values should be t-values if there is only one row in the contrast matrix, which makes sense mathematically: "The F-ratio for the contrast is then given by:
F = G'*inv(C*inv(X'W'*W*X))*C')*G/(J*rvar)
The F is then used to compute a p-value. Note that when J=1, this reduces to a two-tailed t-test."
In my test data, J does equal 1.
Don Hagler wrote:
Are you sure that it is saving t-values in F.mgh and not F-stats?
From: Glenn Lawyer d.g.lawyer@medisin.uio.no To: freesurfer@nmr.mgh.harvard.edu Subject: [Freesurfer] t-value saved values Date: Thu, 20 Sep 2007 15:00:44 +0200
Hi,
I asked about this last week, and had a nice discussion with Doug, but somehow in that we never got the answer to my original question.
When FreeSurfer saves t-values (in F.mgh), what is it actually saving? The numbers in the file do not have a t-distribution (even when the source data for the glm is random noise). Is a transform applied to the t-values before the write, as is done with the p-values?
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