dear fsl experts,
I'm conducting a tbss analysis starting with a simple FA comparison between two groups (controls and patients). I want to include age as a covariate.
I've perused the online documentation and the mailing archives but I'm now unfortunately more confused than before.
Attached are my design files. I would be very happy if someone could take a look at it and tell me if they are ok.
Contrast 1 and 2 are the group comparisons for FA differences
In Contrast 3 and 4 I want to ask where is the slope of Age-vs-FA different between groups
In Contrast 5 and 6 I want to ask for where FA negatively correlates with age for each group seperately.
I have some additional questions:
1) As far as I understood one should add for each group an extra EV into the design matrix (age controls, age patients) to allow for
group specific correlations of age and FA, for example.
If I would only add one extra EV age for all subjects I would assume that the age-FA-dependence is linear and the same across groups.
However what might be the advantage of such an approach - would it be more sensitive to differences?
2) Just to clarify. The -D option in randomise does not replace the demeaning of the covariate in the design matrix. it only takes care of the data itself. Correct?
3) The randomise error "Warning: You have demeaned your data, but at least one design column has non-zero mean" appears in my case although age was properly demeaned.
After demeaning there will be often the situation due to rounding that the mean of the demeaned data will not exactly be 0.
Am I correct that randomise will give this warning even if the mean of the demeaned data is something small like 2.368672e-16?
In my case the demeaned ages have a mean of -0.2 - is that good enough?
4) If I want to manually create my design files I understand /NumWaves, /NumPoints and /Matrix. However what are /PPheights for. What do I need to look after or be careful with here.
Thanks, Michael
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