6 Mar
2012
6 Mar
'12
5:01 p.m.
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 <greve@nmr.mgh.harvard.edu mailto:greve@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 <greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu> <mailto:greve@nmr.mgh.harvard.edu <mailto:greve@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 <greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu> <mailto:greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu>> <mailto:greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu> <mailto:greve@nmr.mgh.harvard.edu <mailto:greve@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. 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--
Douglas N. Greve, Ph.D.
MGH-NMR Center
greve@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