I assume that your 12 columns are: your 6 groups followed by 6 columns representing your covariate.
You must first test that the slopes are not different with an F-test: 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 0 1 -1 0 If you don't find any differences, then you can use the DOSS model. If you find differences, then you have an interaction between your covariate and groups. When this happens, you should consult a statistician if you want to interpret the effects of group as the general rule of thumb is not to interpret the main effects in the presence of an interaction. Using the DOSS model will leave you with 7 regressors. The test for group differences will be: 1 -1 0 0 0 0 0 or 0 1 -1 0 0 0 0 etc. depending on what you are wanting to test. Simply having the covariate in the model corrects for age. A couple of points on covariates - for more details see (http://mumford.fmripower.org/mean_centering/): (A) If you don't mean center, then you will be looking at the group intercepts; (B) If you mean center, then you will be looking at the covariate-adjusted group means; (C) If you mean center by group, then you will be looking at the group means and reducing the variance due to age. For example, if you want to compare old and young subjects, but control for age within each group. A and B will give you the same answer in the DOSS since the slopes of each group are parallel. C will give you a different result. Depending on how you want to interpret your results will effect which method you want to use for mean-centering or not-mean centering the data. In all cases, mean-centering will not effect the slope estimates/significance. Hope this clears up the confusion. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Postdoctoral Research Fellow, GRECC, Bedford VA Website: http://www.martinos.org/~mclaren Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Fri, Nov 23, 2012 at 2:01 AM, 李仕广 <imaging_shiguan...@163.com> wrote: > Dear FreeSurfers: > I hope you can help me out with a problem in disgning my .mtx file. > I want to examine the difference among three groups (HC group, EX group and > PT group). Considing gender, I have six groups as similar as the example > described in http://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf6G0V. But I want > to correct for age. > Then, I have 6 classes (two factors/three l! evels) a nd one covariate > (age). > How can I compare the difference among three groups (HC group, EX group, and > PT group)? > Could you please tall me how to design the contrast matrix with 12 > regressors? > > Thanks a zillion times! > > Best wishes > > shiguangli > > > > > > _______________________________________________ > 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. > _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
