Dear SPM statisticians
I compare fMRI activation maps between 3 Groups (full factorial design). First, I compared the levels in the Factor "Group" by post-hoc t-test (e.g. [1 -1 0]). Now, as my groups differ in age, I want to test developmental trajectories like "early development" or "late development" by contrasts like [1 -1 -1] or [-1 1 1].
I am not sure if these contrasts are statistically legal, because I produce an inballance by comparing the average of two groups against one single group, probably violating some statistical rules? Maybe it is required to recover ballance by weighting the single group by [2 -1 -1]? Further, would you test a linear trayectory by [-1 0 1] or rather by [1 2 3]?
Many thanks for any advise