Hi Martin, Thanks for your reply. I'm not entirely sure what you mean (sorry - I'm new to this!). Do you mean something like this?
For examining an increase between timepoint 1 and 2, but the same between timepoint 2 and 3? fsid fsid-base years years-to-years Day1 base 0 0.4 Day2 base 0.4 0.4 Day3 base 0.8 0 All the best, Jodie Message: 6 Date: Fri, 09 May 2014 16:52:25 -0400 From: Martin Reuter <mreu...@nmr.mgh.harvard.edu> Subject: Re: [Freesurfer] nonlinear models in longitudinal analysis To: freesurfer@nmr.mgh.harvard.edu Message-ID: <536d4009.7030...@nmr.mgh.harvard.edu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Hi Jodie, 'years' should contain the real time between time points (usually, unless you have test-retest where time is irrelevant, or different task that can be ordered as you like). For what you are trying to do you should use linear mixed effects models with a piecewise linear setup. This will allow you to estimate a slope before and after timepoint 2 and compare those slopes. Your design matrix would probably have the column of 1, years and a column with years-years(at tp2 of that subject) if that value is positive 0 otherwise Testing if the coef of the 3rd column is > 0 is asking if the slope increases in the second interval. Not exactly what you want, but close and of course you can test if the slope in interval 1 is different from zero (second column) and if the slope in interval 2 is different from zero ( 0 1 1, ie the sum of the last two columns). Best, Martin On 05/08/2014 03:48 PM, Jodie Davies-Thompson wrote: > Dear Freesurfer experts, > > I am attempting to do a longitudinal analysis of cortical thickness with 3 > timepoints. So far, I have successfully completed the analysis for > long_mris_slopes comparing timepoint 1 to timepoint 2, timepoint 2 to > timepoint 3, and a linear increase for timepoint 1 to 2 to 3. > > However, I am primarily interested in finding regions which show a) an > increase between timepoint 1 and 2, but no difference between 2 and 3, and b) > no difference between timepoint 1 and 2, but an increase between 2 and 3. > > Would the correct way of 'modelling' these two in the qdec file be: > > a) > fsid fsid-base years > day1 base 0 > day2 base 3 > day3 base 3 > > and b) > fsid fsid-base years > day1 base 0 > day2 base 0 > day3 base 3 > > All the best, > Jodie > > - > --- > Jodie Davies-Thompson, Postdoctoral Fellow > Department of Ophthalmology & Visual Sciences > UBC/VGH Eye Care Centre > 2550 Willow Street > Vancouver, BC, V5Z 3N9 > Canada > Tel: 604-875-4111 ext 69003 > - --- Jodie Davies-Thompson, Postdoctoral Fellow Department of Ophthalmology & Visual Sciences UBC/VGH Eye Care Centre 2550 Willow Street Vancouver, BC, V5Z 3N9 Canada Tel: 604-875-4111 ext 69003 _______________________________________________ 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.
