Dear Experts,
I am a little new to design contrast, and matrix design with respect to TBSS
and wonder therefore if I could run some questions past you all.
I am trying to carry out a longitudinal study looking at the effect of
surgery on white matter in the brain. So I basically have patients with DTI
scans before and after surgery (resection of the same part of the brain).
My questions relating to statistical analysis are (assuming that I have 3
patients for the sake of simplicity):
1) My first instinct is to use a paired t-test to analyse the data - however
might there be different cross-subject variance in the different groups as
one has had an extensive surgical operation and the other has not ? In
which case should I be using a 2 sample unpaired t-test
2) In going for the paired t-test model I have created the following design
matrix
(Input 1 - 3 = preop patients, 4-6 = postop patients in same order)
Group EV1 EV2 EV3 EV4
Input 1 1 1 1 0 0
Input 2 1 1 0 1 0
Input 3 1 1 0 0 1
Input 4 1 -1 1 0 0
Input 5 1 -1 0 1 0
Input 6 1 -1 0 0 1
Would this be correct ? Is it necessary in this case to include parameter
estimates 2 to 4 in order to model each subjects mean effect because it
might confound results ?
My design contrast was (looking for increases or decreases in FA after
procedure):
Title EV1 EV2 EV3 EV4
A-B 1 0 0 0
B-A -1 0 0 0
Woud this be correct ?
3) If I then want to carry out a correlation analysis to see how the
location of changes in FA before and after surgery correlate with changes in
some psychology score - how would I do this ? In understand the changes
would have to be demeaned but how would I enter this data into the design
matrix/contrast ?
Thanks.
Mahinda
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