Dear Freesurfers,
I am studying cortical thickness as a function of
age (continuous) and meditation practice (categorical,
level1=Normal, level2=TM). I have three separate cases of
qdec, regression1, regression2 and regression3.
Regression1: Discrete (Fixed Factor) = meditation;
Continuous (Covariate) = age
Regression2: Discrete (Fixed Factor) =
meditation;
Regression3: Discrete (Fixed Factor) = meditation;
Nuisance Factor = age
For the question "Does the average thickness
differ between Normal and TM?", Regression1 and Regression3
have identical result, which is different from Regression2,
i.e., (1 = 3 !=2 ). May I infer from this result the following
about qdec:
1) Regression1 and Regression3 both fit cortical
thickness,y, to the model
y = b0 + age*b1 + meditation*b2 + err = y_hat
+ err
Regression2 fits:
y = b0 + b2*meditation + err = y_hat + err
2) Some function of y_hat goes into the "average
thickness", hence the difference between 2 and the others.
However, what role does "Nuisance Factor" play in
this case?
Thank you very much!
Sincerely,
Ye