External Email - Use Caution Thank you very much.
On Wed, 28 Aug 2019 at 00:01, Greve, Douglas N.,Ph.D. < dgr...@mgh.harvard.edu> wrote: > correct > > On 8/27/2019 11:28 AM, Zhi Li wrote: > > External Email - Use Caution > 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. < > dgr...@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: >> >> External Email - Use Caution >> 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 >> >> >> >> _______________________________________________ >> Freesurfer mailing >> listfreesur...@nmr.mgh.harvard.eduhttps://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer >> >> >> _______________________________________________ >> Freesurfer mailing list >> Freesurfer@nmr.mgh.harvard.edu >> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > _______________________________________________ > Freesurfer mailing > listfreesur...@nmr.mgh.harvard.eduhttps://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
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