Dear SPM community,
Apologies for sending this email again, but I would really appreciate any advice on the following issue:
I am performing a multiple regression in SPM that will examine the main effects of genotype and maltreatment severity and their interaction on grey matter volume.
Genotype has 3 levels (SS, SL, LL), whereas maltreatment is a continuous variable.
I am unsure whether genotype should be included in the multiple regression as 3 dummy variables or one regressor with 3 levels.
Further, should the interaction term be computed by multiplying the mean-centred scores of genotype and maltreatment?
Any advice would be greatly appreciated.
Kind regards,
Micaela Adams
University of Birmingham
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