I want to test the relationship between cortical thickness and two continuous covariates (interaction) while controlling for one categorical variable. In QDEC, I can only choose one covariate, so I created a new variable that is a dot product (covariate1*covariate2) of the two to represent interaction. After analyzing, when I look at the data points at a single voxel, there is no visual indication of interaction. There also is no fitted line. 

Am I utilizing QDEC correctly? 

Thank you for your help!

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