Dear Freesurfer experts,
I have followed the LongitudinalTutorial with two time points + two groups and now, after looking at the output of the qdec analysis, I just want to make sure I haven't made a sign error as for the control group the difference is as expected around 0 but for the experimental group I see negative values (= thinning?) all over the place which seems weird, just want to make sure I got this right:
I set the time variable to 0 for the baseline scan and to 1 for the after-intervention-scan. Is it correct that for do-rate the calculation is then only <thickness of tp2> - <thickness of tp1> and not the other way around? I ask because in the long_mris_slopes -help it says that do-rate would yield the "thinning in mm/time" and if this means that a positive value stands for thinning then this would contradict what I understood from the Tutorial where it says for the rate "thick2-thick1" is calculated, so a positive value should mean "thickening".
Also, how does long_mris_slopes know which one is tp1 and which one is tp2? I assume it takes the first entry in the fsid column of long.qdec.table.dat as tp1 and the second entry below as tp2?
Hm, I see no other possibilities of making a sign error during the whole process, did anyone report a similar issue?
Thanks for your help! Best, Vincent