Dear Experts,
I have 2 groups - controls (22 subjects) and patients (19 subjects) with mean ages that are not significantly different, though ranges/median are. I want to compare cortical thickness between groups while allowing for age (age as nuisance factor) but also want to see if there is an interaction between age and group (age as continuous variable). I have demeaned the age across both controls and patients and then used qdec.
1) Using DODS and demeaned ages I find that there is an age-gp interaction such that patients have quicker thickness decline with age than controls. This survives FDR 0.05 and monte carlo VWP 0.005 CWP 0.05. Is this finding valid given that I used demeaned ages (rather than absolute ages) to look for interaction or does it make no difference ?
2) When I come to look to see whether there is a gp difference in thickness I enter age as a nuisance variable in qdec (as opposed to continuous variable above) and use DODS with demeaned age variable across gp. Am I safe to interpret my findings normally given age-gp interaction or is is not safe - if it is not safe how can I look for gp differences or it ok to interpret results provided gp differences are not in regions where I see an interaction ?
Thanks.
Mahinda