It means that any comparison depends on age. Eg, if you compare them at an age where the lines cross, you will see no effect. If you compare them at age=0 (what you were doing before), then you see a big effect. Some statisticians will say that you cannot do the comparison in the presence of an interaction. dogu
Allie Rosen wrote: > So if my findings show that each group ages differently, does that > mean I can't compare them at all because I can't regress out age? Or > is there some way of comparing the groups despite this difference in > aging patterns? > > Thanks again, > Allie > > On Tue, Mar 6, 2012 at 4:44 PM, Douglas N Greve > <gr...@nmr.mgh.harvard.edu <mailto:gr...@nmr.mgh.harvard.edu>> wrote: > > It might, don't know > > Allie Rosen wrote: > > Hi Doug, > > Yes, I think there actually may be a topologic defect in this > area, although not directly where the cluster is (attached). > Could this be the cause of the thickness result? If I go back > and try to repair the defect, will that make the results more > typical and, more importantly, correct? > > Thanks again, > Allie > > > > On Tue, Mar 6, 2012 at 3:57 PM, Douglas N Greve > <gr...@nmr.mgh.harvard.edu <mailto:gr...@nmr.mgh.harvard.edu> > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>>> wrote: > > Hi Allie, yes, it means that each group is affected > differently by > age. The controls are increasing their thickness with age > in this > area (which seems strange, usually you expect general decreases > with age). You might check the quality of the surfaces in > this area. > doug > > Allie Rosen wrote: > > Hi Doug, > > Thanks for the response. I've actually already run an > interaction contrast and the same area is still significant > (attached). I'm not sure how to interpret this either, > unfortunately. Does it say that both groups are affected > significantly differently by age? Can you tell me what it > means? I'm having problems trying to do this myself. > > Thanks very much, > Allie > > On Tue, Mar 6, 2012 at 3:47 PM, Douglas N Greve > <gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu> > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>> > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu> > > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>>>> wrote: > > Allie, to be more precise, you have selected > control<patient > regressing out the effects of age. This second part > is the key. > If you were to trace the regression lines back to > age=0 (the > meaning of "regressing out age"), then > control<patient by a > wide > margin. But you have a problem here in that the > regression > lines > are crossed. This means that you have an interaction > between age > and patient (no interaction means that the lines > would be > parallel). An interaction means that you cannot > assess whether > there is a difference in the thickness because this > difference > changes depending upon the age. One thing you can > try is to > actually compute an interaction contrast (something > like [0 0 1 > -1]). If it is significant in this area, then > there's not > much you > can do (though it may be interesting in itself). If > it is not > significant, then you can switch to a DOSS model which > forces the > lines to be parallel and makes the distance between the > lines be > independent of age. Does this make sense? > doug > > > > Allie Rosen wrote: > > Hi All, > > I'm having trouble reconciling the cluster > colours with the > graph that can be made when you load the group > descriptor file > and click on a point. I've attached an example. > In this study, I'm comparing thickness in a > patient and > control group, with age regressed out. The contrast > I've used > means that blue means control<patient. Therefore, > patients are > thicker than controls. But in the graph, > patients don't > seem > to be thicker. I've seen the same results with > red clusters > (i.e. they should mean that controls are thicker but > the graph > shows otherwhise). Please let me know how I can > interpret my > findings given the graph. Which group is > actually thicker? > > Thank you, > Allison > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > <mailto:Freesurfer@nmr.mgh.harvard.edu> > <mailto:Freesurfer@nmr.mgh.harvard.edu > <mailto:Freesurfer@nmr.mgh.harvard.edu>> > <mailto:Freesurfer@nmr.mgh.harvard.edu > <mailto:Freesurfer@nmr.mgh.harvard.edu> > > <mailto:Freesurfer@nmr.mgh.harvard.edu > <mailto: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 > <mailto:gr...@nmr.mgh.harvard.edu> > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>> > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu> > > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>>> > Phone Number: 617-724-2358 <tel:617-724-2358> > <tel:617-724-2358 <tel:617-724-2358>> > <tel:617-724-2358 <tel:617-724-2358> <tel:617-724-2358 > <tel:617-724-2358>>> Fax: 617-726-7422 <tel:617-726-7422> > <tel:617-726-7422 <tel:617-726-7422>> > <tel:617-726-7422 <tel:617-726-7422> > <tel:617-726-7422 <tel:617-726-7422>>> > > > Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting > <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> > <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> > <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> > FileDrop: > www.nmr.mgh.harvard.edu/facility/filedrop/index.html > <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> > > <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> > > <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> > > > > > 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. > > > > > > ------------------------------------------------------------------------ > > > -- Douglas N. Greve, Ph.D. > MGH-NMR Center > gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu> > <mailto:gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>> > Phone Number: 617-724-2358 <tel:617-724-2358> > <tel:617-724-2358 <tel:617-724-2358>> Fax: 617-726-7422 > <tel:617-726-7422> > <tel:617-726-7422 <tel:617-726-7422>> > > Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting > <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> > <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> > FileDrop: > www.nmr.mgh.harvard.edu/facility/filedrop/index.html > <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> > <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> > > > > > ------------------------------------------------------------------------ > > > -- > Douglas N. Greve, Ph.D. > MGH-NMR Center > gr...@nmr.mgh.harvard.edu <mailto:gr...@nmr.mgh.harvard.edu> > Phone Number: 617-724-2358 <tel:617-724-2358> Fax: 617-726-7422 > <tel:617-726-7422> > > Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting > <http://surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> > FileDrop: www.nmr.mgh.harvard.edu/facility/filedrop/index.html > <http://www.nmr.mgh.harvard.edu/facility/filedrop/index.html> > > -- 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: www.nmr.mgh.harvard.edu/facility/filedrop/index.html _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
