Re: [Freesurfer] How to design the contrast matrix with 12 regressors?

-- Michael Harms, Ph.D. ----------------------------------------------------------- Conte Center for the Neuroscience of Mental Disorders Washington University School of Medicine Department of Psychiatry, Box 8134 660 South Euclid Ave. Tel: 314-747-6173 St. Louis, MO 63110 Email: mharms@wustl.edu On 11/26/12 4:32 PM, "MCLAREN, Donald" mclaren.donald@gmail.com wrote: >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_shiguangli@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