Thank you Martin! For only one group, would the design matrix be the same but intercept=Group=all ones?
Yij = ß1 + ß2*tij +ß3*Group + ß4*Group *tij + ß5*Genderi + ß6*Agei + ß7*ICVi+ b 1i + b2i*tij + eij Regards, Yolanda 2013/5/28 Martin Reuter <mreu...@nmr.mgh.harvard.edu> > Hi Yolanda, > > since you have the intercept you don't need both group1 and group2 (as one > is 1 - the other), so drop beta 5 and 6 terms. > If you want to test if the slope is different between the groups differ > just look at beta4 which is checking if slope1-slope2 different from zero. > If it is negative slope2 is larger, if positive slope1 is larger. Note that > if your slopes are negative, e.g. atrophy, then larger means closer to zero > = less atrophy. > > Best, Martin > > > On 05/28/2013 11:57 AM, Yolanda Vives wrote: > > Dear FreeSurfer experts, > > I am trying to use the LME toolbox to analyse the hippocampal change over > time in a group and also among 2 groups (2 scans per subject). > > When I estimate the parameters with lme_fit_FS I become a warning saying > that the matrix is singular and my results are NaN. I guess that my design > matrix X is not correct, can you please help me? > > I have followed the example from the wiki. My model for the two groups > study is the following: > > Yij = ß1 + ß2*tij +ß3*Group1 + ß4*Group1 *tij + ß5*Group2 + ß6*Group2 *tij+ ß > 7*Genderi + ß8*Agei + ß9*ICVi + b1i + b2i*tij + eij > > 1) intercept (all ones) > > 2) time (tij) . Here I have 0s for the first scan of each subject and a > number in years for the second scans. Is it correct? > > 3) one for Group1 and zero otherwise > > 4) colum 3) .* time > > 5) one for Group2 and zero otherwise > > 6) colum 5) .* time > > 7) Gender > > 8) Age of the subject at each scan moment. > > 9) ICV (converted to liters). > > Thank you in advance, > Yolanda > > > _______________________________________________ > Freesurfer mailing > listfreesur...@nmr.mgh.harvard.eduhttps://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > -- > Martin Reuter, Ph.D. > Assistant in Neuroscience - Massachusetts General Hospital > Instructor in Neurology - Harvard Medical School > MGH / HMS / MIT > > A.A.Martinos Center for Biomedical Imaging > 149 Thirteenth Street, Suite 2301 > Charlestown, MA 02129 > > Phone: +1-617-724-5652 > Email: > mreu...@nmr.mgh.harvard.edu > reu...@mit.edu > Web : http://reuter.mit.edu > > 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. >
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