---------- Forwarded message ----------
From: Paul Macey <[log in to unmask]>
Date: 2015-10-20 18:00 GMT+02:00
Subject: [FSL] FIRST beta values
To: [log in to unmask]


I'm running a 4-variable analysis in FIRST (one group, three covariates), and I have significant results with the F-test. I now want to see the direction and size of the effects; in a typical model I would look at the betas/model coefficients, but these are not available with the vertex analysis. I see in the documentation (http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FIRST/UserGuide#randomise) the recommended approach is to do a t-test, or look at the actual values with the FSL timeseries tool (or by extracting the data).

The problem with the t-test is that I only get one result ("....tstat1") even though I have 4 contrasts (see design.con below), and I get 4 F stats. I'm not sure which contrast tstat1 refers to, and furthermore I'm not sure how to translate the t statistic for this 4-variable model to an effect size. For a t statistic from a 2-sample t test multiplying by std/sqrt(N) would work - is it the same with a 4 variable model?

Similarly, the issue with extracting the individual values is that I while I could look for group differences, I cannot account for the 3 other variables in the model without running another analysis on the extract values, which is possible but no ideal.

So my questions are:
- is it possible to obtain the betas?
if no,
- how can I obtain the t statistic maps for my 4 contrasts?
- is there a way to translate the t statistic in this 4-variable model to a non-standardized effect size?

Any guidance appreciated!

-------------------
design.con
/ContrastName1  group mean
/ContrastName2  TIVvol mean
/ContrastName3  age mean
/ContrastName4  sex mean
/NumWaves       4
/NumContrasts   4
/PPheights              1.552420e+00    1.719039e+00    9.529518e+01    1.722726e+00
/RequiredEffect         0.673   0.673   0.560   0.417

/Matrix
1.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00
0.000000e+00 1.000000e+00 0.000000e+00 0.000000e+00
0.000000e+00 0.000000e+00 1.000000e+00 0.000000e+00
0.000000e+00 0.000000e+00 0.000000e+00 1.000000e+00