Several potential problems. It makes the statistics non-gaussian and the abs(ces) becomes noise dependent (and so there will be a noise dependent bias). This makes interpretation more difficult (eg, is a difference between groups a difference in ces or a difference in noise?). The non-gaussianity could be addressed with a permutation test.
doug On 01/08/2014 07:11 PM, sha...@nmr.mgh.harvard.edu wrote: > Exactly! I need ces and cesvar files for a second order analysis. More > specifically, I need to subtract this absolute map from another map. > Would you be more specific about what I should check when I am doing this > second order analysis? > > > >> You can do it to the sig file (the p-value already assumes a two-sided >> test). Why do you want to do it with the ces and cesvar files? If you >> are planning to use the abs(ces) for a higher level analysis then you >> need to be very careful as you can get some unintended effects >> doug >> >> >> >> >> On 01/07/2014 03:46 PM, SHAHIN NASR wrote: >>> Hi, >>> I have two related questions. I have two conditions (e.g. C1 and >>> C2) and I want to generate a map that shows all voxels that can >>> differentiate C1 from C2 _irrespective_ of whether C1>C2 or C1<C2. I >>> think it can be done easily by: >>> >>> mris_calc -o sig.nii abs_sig.nii abs >>> >>> If I am right, then: >>> 1) Can I apply the same command to ces.nii file to have the absolute >>> value of the effect size (i.e. abs(C1-C2 effect size)) or should I >>> consider other factors? >>> 2) Do I need to apply the same routine to cesvar? >>> >>> Thanks >>> -- >>> Shahin Nasr >>> >>> PhD in Cognitive Neuroscience >>> Martinos Imaging Center, MGH >>> Harvard Medical School >> -- >> 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: https://gate.nmr.mgh.harvard.edu/filedrop2 >> www.nmr.mgh.harvard.edu/facility/filedrop/index.html >> Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ >> >> >> > -- 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: https://gate.nmr.mgh.harvard.edu/filedrop2 www.nmr.mgh.harvard.edu/facility/filedrop/index.html Outgoing: ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/ _______________________________________________ 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.
