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I see. Hence I cannot set the contrast of interaction between two covariates if I do not create the interaction term by myself, right?

On Tue, 27 Aug 2019 at 10:53, Greve, Douglas N.,Ph.D. <DGREVE@mgh.harvard.edu> wrote:

Actually, the wiki page is not quite right. For a simple interaction between age and weight, you would need to create a new regressor = age*weight, then set the contrast to [0 0 0 0 0 0 1]. If you want an interaction between group, age, and weight, then you would need two new regressors, age*weight for group1 and age*weight for group2, then the contrast will be [0 0 0 0 0 0 1 -1]. I would suggest demeaning both age and weight prior to computing the product.

On 8/27/2019 10:12 AM, Zhi Li wrote:

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Dear Freesurfer experts,

May I ask how to understand the contrast of interaction between covariates in the example https://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G2V? The interaction of group x age x weight is:

[0 0 1 -1 0 0

 0 0 0 0 1 -1]

Why the interaction between group and weight should be put in the second row (what does each row mean in the F-contrast matrix)?

If there are three levels in the factor one (three groups), I guess the interaction of group x age x weight should be:

[0 0 0 1 -1 0 0 0 0

 0 0 0 0 1 -1 0 0 0

 0 0 0 0 0 0 1 -1 0

 0 0 0 0 0 0 0 1 -1] 

Is this correct? If not how should I set this contrast?

In addition, if I would like to know the interaction between age and weight without group, how should I set the contrast then?

Looking forward to your kind suggestions.

Thanks and best wishes,

Lizhi

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