On 11/04/2015 05:10 AM, Erik Lindberg wrote: > > Dear Freesurfer experts, > > I know this has been an ongoing topic in the mailing list – however I > would like to raise the issue one more time. In the choice between > doss and dods – I usually check if there is a significant interaction > and if so my choice is dods. > > As I understand it you do not recommend to use doss if an interaction > is present. Is the reason for this that I would “over-correct” for the > variable in one group while “under-correcting” for the same variable > in the other group? > If the interaction is present, then it (almost) makes no sense to run the DOSS because you would be looking for a difference in group intercepts. With an interaction, the difference in intercepts changes depending upon where the intercept is measured (eg, age=0 vs age=70). If there is an interaction, then the thickness vs age plots for the two groups MUST intersect at some age. If you test for a difference in groups at that age, you will not find one. If you test younger you will see a sign flip relative to older. The DOSS model is not appropriate because it assumes/forces the plots to be parallel. > > It is some time tempting to present result both from a dods and a doss > – to tell where the interaction is present, and where difference are > present while controlling for the variable > > But if I understand Doug´s previous comments on this – this would not > be a correct approach? > Not sure that would work. Why not just test for a difference between slopes? > > Thanks > > Erik > > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
-- Douglas N. Greve, Ph.D. MGH-NMR Center gr...@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422 Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting FileDrop: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer The information in this e-mail is intended only for the person to whom it is addressed. If you believe this e-mail was sent to you in error and the e-mail contains patient information, please contact the Partners Compliance HelpLine at http://www.partners.org/complianceline . If the e-mail was sent to you in error but does not contain patient information, please contact the sender and properly dispose of the e-mail.
