Hello AFNI team (probably Gang :)-D),
If you are performing a t-test (3dttest++ for example) between two groups of old and young age.
Would you add age as a covariate?
I would bet not since age is not independent of the 2 variables tested right?
I just wanted to be sure,
Note that the default for 3dttest++ is -center DIFF, though I guess you should specify that anyway.
In the case of -center DIFF, each group will have any covariate means removed separately. So here the age covariates should be orthogonal to the grouping terms. At least that would apply to 3dttest++…
Does that seem reasonable?
yes, thank you very much for your answer!