Hi there,
I'm doing an across-subjects analysis of a mixed fMRI design. I just
wanted to check on a couple of things and hope you can help.
I've got n=15 in one group and n=16 in a second group, plus there are
two repeated measures factors (one with 2 and one with 3 levels). What
I'm planning on doing is modeling the repeated measures effects in the
EVs as shown below with two example subjects in the last two EVs (if
the columns don't line up properly, it's replicated in the attached
txt file):
EV1 EV2 EV3 EV4 EV5 EV6 EV7
subj cope A1>A3 A2>A3 B2>B1 A1*B A2*B subj1 subj2...
1 A1B1 1 0 -1 -1 0 1 0
1 A1B2 1 0 1 1 0 1 0
1 A2B1 0 1 -1 0 -1 1 0
1 A2B2 0 1 1 0 1 1 0
1 A3B1 -1 -1 -1 1 1 1 0
1 A3B2 -1 -1 1 -1 -1 1 0
2 A1B1 1 0 -1 -1 0 0 1
2 A1B2 1 0 1 1 0 0 1
2 A2B1 0 1 -1 0 -1 0 1
2 A2B2 0 1 1 0 1 0 1
2 A3B1 -1 -1 -1 1 1 0 1
2 A3B2 -1 -1 1 -1 -1 0 1
.
.
.
Then, in order to get the group effects, I need to make contrasts
using these subject EVs, right? So it would look like this (with only
2 subjects in EVs 6 and 7)):
EV1 EV2 EV3 EV4 EV5 EV6 EV7
grp1 > grp2 0 0 0 0 0 1 -1
grp2 > grp1 0 0 0 0 0 -1 1
And then to see if there are group differences in the repeated
measures effects, something like this (for grp1>grp2 in EVs 6 and 7):
A1>A3 1 0 0 0 0 1 -1
A2>A3 0 1 0 0 0 1 -1
B2>B1 0 0 1 0 0 1 -1
A1*B 0 0 0 1 0 1 -1
A2*B 0 0 0 0 1 1 -1
Yes? No? Utter hogwash?
Finally, I was thinking the group contrast should sum to zero, but it
won't with different numbers in each group unless I compensate. So
let's say there are 3 subjects total, 1 in grp1 and 2 in grp2. For
grp1>grp2, would I enter [2 -1 -1] for the three subject EVs to
compensate, or is this unnecessary?
Cheers, and thanks in advance for your input,
Heather
--
Heather L. Urry
Department of Psychology
Tufts University
490 Boston Avenue
Medford, MA 02155
email: [log in to unmask]
phone: 617-627-3733
fax: 617-627-3181
EVs
EV1 EV2 EV3 EV4 EV5 EV6 EV7
subj cope A1>A3 A2>A3 B2>B1 A1*B A2*B subj1 subj2...
1 A1B1 1 0 -1 -1 0 1 0
1 A1B2 1 0 1 1 0 1 0
1 A2B1 0 1 -1 0 -1 1 0
1 A2B2 0 1 1 0 1 1 0
1 A3B1 -1 -1 -1 1 1 1 0
1 A3B2 -1 -1 1 -1 -1 1 0
2 A1B1 1 0 -1 -1 0 0 1
2 A1B2 1 0 1 1 0 0 1
2 A2B1 0 1 -1 0 -1 0 1
2 A2B2 0 1 1 0 1 0 1
2 A3B1 -1 -1 -1 1 1 0 1
2 A3B2 -1 -1 1 -1 -1 0 1
.
.
.
Contrasts
EV1 EV2 EV3 EV4 EV5 EV6 EV7
grp1 > grp2 0 0 0 0 0 1 -1
grp2 > grp1 0 0 0 0 0 -1 1
A1>A3 1 0 0 0 0 1 -1
A2>A3 0 1 0 0 0 1 -1
B2>B1 0 0 1 0 0 1 -1
A1*B 0 0 0 1 0 1 -1
A2*B 0 0 0 0 1 1 -1
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