SPM List,
We ran a repeated measures ANOVA (flexible factorial) in SPM to test for
interactions between two experimental factors.
We have 3 groups (n1=4, n2=16, n3=6) and 2 conditions (pre- and post-).
Therefore, we have a total of 52 scans.
We have included three factors in the model: SUBJECT, GROUP and CONDITION.
SUBJECT: independence-yes, variance-equal
GROUP: independence-yes, variance-unequal
CONDITION: independence-no, variance-equal
We specified three main effects (one for each factor: SUBJECT, GROUP and
CONDITION) and the interaction between GROUP and CONDITION.
We have the following questions
1. Is it correct to test for the interaction in this model (that is, using
the flexible factorial, mixed effects design)?
2. If we use this model to test for the interaction, we are not sure how
to extend the contrast weights from Glascher and Gitelman for a 3 x 2
design.
Is this correct?
Ho = G1C1 - G1C2 = G2C1 - G2C2
G2C1 - G2C2 = G3C1 - G3C2
F Contrast
zeros(1,24) 0 0 0 0 0 1 -1 -1 1 0 0
zeros(1,24) 0 0 0 0 0 0 0 1 -1 -1 1
3. If we test the interaction with the flexible factorial, and there is no
interaction between the factors (GROUP x CONDITION),
is it correct to run a full factorial model to test for main effects for
GROUP and CONDITION including the interaction in the model
or should we average the groups and conditions and run a one-way ANOVA and
a paired t-test separately to find the main effects for GROUP and CONDITION?
Thank you,
Bedda
