On 6 Oct 2010, at 15:38, Rodrigo Perea wrote:
Hello,
I am new in this field and I am very skeptical with the different matrix designs I've tried when I wanted to analyze a linear relationship case. I want to check the correlation between FA and age in one time point in individuals of the same group with sex as a covariate.
For example, lets suppose we have 10 individuals, my guesses were:
1. I demeaned the age and set up two contrasts.
Design.mat: In the first column it shows that all belong to Group1
Not quite - in the actual design matrix the first EV as shown below is modelling the overall group mean.
, 2nd colum is the demeaned age and the 3rd column my covaraite sex.
You would normally demean the 3rd EV as well - but in this case it won't matter.
/NumWaves 3
/NumPoints 47
/PPheights 1.000000e+00 2.558000e+01 1.000000e+00
/Matrix
1 -2.4 1
1 -2.8 1
1 5.7 0
1 -4.8 1
1 3.4 0
Desing.con: Since the 2nd EV is the one I want either a positive or negative relation, I ended doing this...
/Matrix
0 1 0
0 -1 0
Correct
Or my second guess
2. I didnt set up the group values since all will be assigned to the same group. Thus, 1st column is the demeaned age and 2nd the covariate sex.
No - the first example is correct - note that the part of the Glm GUI where you enter group membership may be used in the modelling, but is NOT part of the design matrix.
Cheers.
Design.mat:
/Matrix
-2.4 1
-2.8 1
5.7 0
-4.8 1
3.4 0
Design.con
/Matrix
1 0
-1 0
Thanks in advance!
Rodrigo Dennis Perea
Neuro-Imaging Research Assistant
[log in to unmask]Alzheimer's & Aging Program
The University of Kansas - School of Medicine