Try a design of this form
1 0 34 0
1 0 22 0
0 1 0 15
1 0 25 0
0 1 0 29
Etc..
And test the interactions with [0 0 1 -1] or [0 0 -1 1].
Best regards,
-John
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
On Behalf Of Nicolas Cherbuin
Sent: Friday, June 22, 2007 8:20 AM
To: [log in to unmask]
Subject: [SPM] interactions with continuous variables (VBM with SPM2)
2nd attempt
Sorry for the repeat post but I did not get any answer.
Dear SPMers,
I have spent the last couple of hours searching the list for information
on interactions with continuous variables in vbm (SPM2) but I am still
very confused.
I have two groups (A and B) and a covariate C. I would like to test the
interaction between groups and the continuous variable C but I am not
sure about the correct way to go about it.
I have set up the following variables
GroupA GroupB CovariateC
1 0 34
1 0 22
0 1 15
1 0 25
0 1 29
Etc..
I then was not sure whether to test the interaction between groups and
covariates I should simply multiply groupA by covariateC to produce
AxCInteraction and then use the following contrast
GroupA GroupB CovariateC AxCInteraction
0 0 0 1
Or whether I should set up two interaction variables for each group
variable and test the interaction with the following contrast
GroupA GroupB CovariateC AxCInteraction BxCInteraction
0 0 0 1 -1
Your assistance would be greatly appreciated
Nic
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