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

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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

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