I have a question regarding linear contrasts output from 3dMVM in comparison to output from 3dttest++
I have a 2x3 within subject design.
factor1: with 2 levels= pre, post
factor2: with 3 levels= one, two, three
We ran 3dMVM and setup several glt tests. One of the tests involved collapsing across 3 levels for Factor2 for level pre of Factor1
-gltLabel 1 ‘pre_Factor123’ -gltCode 1 ‘factor2 : .333One.333two .333three factor1 : 1pre’ \
The second contrast involved collapsing across 3 Factor2 for level post of Factor1
-gltLabel 2 ‘post_Factor123’ -gltCode 2 ‘factor2 : .333One .333two .333three factor1 : 1post’ \
The third contrast tried to find the difference between pre-post factor1, collapsed across all 3 levels of factor2.
-gltLabel 3 ‘Factor123_pre-post’ -gltCode 3 ‘factor2 : .333One .333two .333three factor1 : 1pre -1*post’ \
Is this correct?
We collapsed across all 3 levels for factor 2 and ran a simple paired ttest with 3dttestt++ to cross-check. We get pretty much the same result for means and mean differences, but the t and p-values are different? Why is that the case?
I think it might be the degrees of freedom.
We have 36 subjects per each of the 3 levels for Factor2. The statpar from 3dttest++ is 107. The statpar for the bucket of each of the 3 contrasts described for 3dMVM is 35.
Why is this?