I am struggling with wrong Null_contributions outputs of FBA for some time and couldn’t find a answer in the forum.
My matrix for fixelcfestats on smoothed FD images is:
ones demeaned variable1 demeaned variable2
1 0.2 0.1
1 0.1 -0.09
1 -0.3 0.01
My contrast is 1 0 0, which I think reflects mean FD for the whole group (as I demeaned the covariates).
The outputs of Z-values or t-values seem reasonable, as mean FD should be positive across the brain. However, Null_contributions and 1-fwe outputs seem incorrect.
Z-values range from 13 to 35.4
t-values: 15.8 - 228.1
abs-effects: 0.1 - 1.4
Null_contributions: 0 - 5000 (one fixel is 5000, others 0)
1-fwe: 0.0002 - 0.0002
I tried other contrasts for the 2 covariates (e.g. 0 1 0, 0 0 1), the values of null_contributions outputs are all within reasonable range (lower than 10).
What could be the possible reason for the incorrect outputs when I look at the global intercept? Any suggestion would be highly appreciated!