It depends on the test that you want to make. By using abs, your hypothesis is that there is a difference between the groups but you don't care about the direction of the difference. The mean thickness test is a different hypothesis, so it's not too surprising that you are getting different results. Both can be accurate.
doug Joana Braga Pereira wrote: > Dear Doug, > > Thanks for the reply. > > Would you recommend using pos or neg sign, instead of abs for a group > comparison? > > Are my vertex-wise results showing true differences between controls > and patients even if the mean thickness of the clusters is not > statistically different? Or should i just consider the clusters that > have statistically different mean thickness between groups? > > Many thanks > > 2010/8/3 Douglas N Greve <gr...@nmr.mgh.harvard.edu > <mailto:gr...@nmr.mgh.harvard.edu>> > > It might be, it depends on how you defined the sign of the > voxel-wise threshold. If you used "abs", then it is certainly > possible. > > doyug > > Joana Braga Pereira wrote: > > Dear all, > > I would like to ask you whether there should be an exact > correspondence between vertex-wise thickness and mean > thickness of clusters. > > I'll explain you my problem. > I did an analysis with 2 groups and 2 covariates. Then i run > the mri_glmfit script and found clusters of greater thickness > in controls compared to patients. When i extracted the mean > thickness of these clusters and did a two sample t test i > observed that the controls had greater thickness than > patients, however not statistically significant, even after > including in this analysis the 2 covariates. > > Is this normal? How does freesurfer uses the covariates when > performing an analysis? > > Many thanks > > ------------------------------------------------------------------------ > > _______________________________________________ > Freesurfer mailing list > 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> > Phone Number: 617-724-2358 Fax: 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 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.
