The mailing list archives have already been an enormous help in
working with this cumbersome design, so I'm very hopeful that you
fine folks can help me out here!
Experiment setup:
Mixed ANOVA - repeated measures with two groups (Control group and
Intervention group)
Timeline:
Pre-treatment scan / 10 weeks of treatment / post-treatment scan at
10 week mark
QUESTION ONE-
Goal:
To find the difference scores between Scan 1 and Scan 2 for each
subject and to compare the average difference between the two groups
of subjects
To answer the question, "Does group membership correspond to
different levels of change in activity?"
Approach:
1) Fixed-effects analysis, one for each group, calculating subject
differences as contrasts (Scan2-Scan1 and Scan1-Scan2)
2) Mixed-effects analysis, using inputs from both groups, with one
EV for each group and contrasts looking at group averages, Control
> Intervention, and Intervention > Control
I did NOT model the subject means in step 1 here - should I? Since
I'm not making any inferences at that level, I don't know whether
it's necessary, and since I've done the next analysis with
mixed-effects, I figure that accounts for subject as a random
effect.
However, I ask because the subject means ARE modeled in the single-group
paired difference, whereas the instructions for randomise
advise a simple subtraction approach.
******************************
QUESTION TWO - This question is illustrated in the attached
.pdf, but in short:
I have a subject who has two scans that each consist of multiple
runs, and I want to get an estimate of that subject's average
activation.
I've tried two approaches:
1) Average the runs for each scan separately and then average those
two scan averages to put in a two-group model (similar to this)
2) Put together all the runs for a single subject into one model and
calculate an average from that to put in a two-group model
Can someone tell me why the outcomes of those two analyses are
different?
What additional adjustment or calculation is being performed there?
Is there a compelling methodological reason to do it one way or the
other?
Thank you so much, and happy holidays!
-Katie
