Thanks Doug for your reply. I forgot that the number of columns is number of classes by number of covariates +1.
So, I only want you to confirm me that the extra zero that I've seen in the FSwiki example is an erratum (see contrasts 7 and 8 in Four Groups (2 Factors, Two Levels), One Covariate. DODS only. QDEC-able) and that I don't have to add any extra zero per covariate when testing the interaction effect. Many thanks again. Jose ----- Mensaje original ----- De: Douglas N Greve <gr...@nmr.mgh.harvard.edu> Fecha: Martes, 18 de Enero de 2011, 16:53 Asunto: Re: [Freesurfer] contrast matrix for 2 discrete factors and 3 continuous variables A: Jose Luis Cantero Lorente <jlcan...@upo.es> CC: freesurfer@nmr.mgh.harvard.edu > Hi Jose, with 6 classes and 3 continuous variables, I think you > should have 24 regressors, which means 24 columns in the > contrast matrix. Yours only has 18. Otherwise the contrast to > test between B and C is correct (just need to add 6 zeros). For > the interaction, you would need > > 0 0 0.5 -0.5 -0.5 0.5 0 0 .... > > doug > > Jose Luis Cantero Lorente wrote: > > > >Dear Freesurfers, > > > > > >My experimental design includes 2 discrete factors and 3 > continuous variables: > > > > > >Discrete factors: > > > >- Diagnosis with three > levels (A, B, C) > > > >- Gender with two levels > (Males, Females) > > > > > >Continuous variables: > > > >- Mean cortical thickness > in the left hemisphere (CTLH) > > > >- Age > > > >- Performance in a memory > test (TASK) > > > > > >The FSGD file would be as follows: > > > > > >GroupDescriptorFile 1 > > > >Title Thickness-subj > > > >Class AMale > > > >Class AFemale > > > >Class BMale > > > >Class BFemale > > > >Class CMale > > > >Class CFemale > > > >variables ZCTLH ZAGE > > > >Input control_01 AMale -1.04398 0.24789 > > > >Input control_02 AMale 1.22853 -0.72343 > > > >Input control_03 AFemale .13773 -1.37097 > > > >Input control_04 AFemale .41043 0.24789 > > > >Input mci_01 BMale -.77128 0.24789 > > > >Input mci_02 BMale -2.40749 0.73354 > > > >Input mci_03 BFemale -.86218 -1.0472 > > > >Input mci_04 BFemale .50133 0.23777 > > > >Input ad_01 CMale -.75678 0.22789 > > > >Input ad_02 CMale -2.88749 0.63047 > > > >Input ad_03 CFemale -.96015 -1.4452 > > > >Input ad_04 CFemale .80712 -0.37567 … > > > > > >We are assuming different offsets and different slopes (DDOS). > > > > > >The following contrast would test the null hypothesis whether > there is a difference between B and C (T-test) regressing out > the effect of the remaining variables. > > > > > >0 0 0.5 0.5 -0.5 -0.5 0 0 0 0 0 0 0 0 0 0 0 0 > > > > > >Could you help me to build the contrast for testing > interactions in the above experimental design? In the FSwiki > examples, you add one column when you have two factors with two > levels each, and one continuous variable. I am not sure how many > columns should I add in our particular case and why. > > > > > >Null hypothesis: is there an interaction between diagnosis (B > > C) and gender regressing out the two remaining variables? > Assuming that you add one column per variable that is regressed > out, the contrast would be as follows: > > > > > >0 0 0.5 0.5 -0.5 -0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 > > > > > >Is this correct? Any help will be very appreciated. > > > > > >Many thanks in advance. > > > > > >Best regards, > > > >Jose > > > > > >--- > >Jose L. Cantero, Ph.D. > >Laboratory of Functional Neuroscience > >Department of Physiology, Anatomy and Cell Biology > >University Pablo de Olavide > >Ctra. de Utrera, Km.1 > >41013 - Seville > >- Spain - > >Phone: +34 954 977433 > >Fax: +34 954 349151 > >Email: jlcan...@upo.es > >http://www.upo.es/neuroaging/en/ > >---------------------------------------------------------------- > -------- > > > >_______________________________________________ > >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: 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. > --- Jose L. Cantero, Ph.D. Laboratory of Functional Neuroscience Department of Physiology, Anatomy and Cell Biology University Pablo de Olavide Ctra. de Utrera, Km.1 41013 - Seville - Spain - Phone: +34 954 977433 Fax: +34 954 349151 Email: jlcan...@upo.es http://www.upo.es/neuroaging/es/
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