Hello,

I am having some troubles using QDEC to analyze the thickness changes in my group study and I was hoping someone may be able to give me a hand. The steps I have done are as follows:

I am working on a study looking at 40 subjects scanned at two time points (base and 6 months after). I have followed all the steps for longitudinal processing for each subject (cross,base, and longitudinals) and made edits where they were necessary to the base. I then entered all 40 subjects into a QDEC table as follows

fsid fsid-base time gender age crosstpN1 base1 0 female 20 crosstpN2 base1 6 female 20 .......... continues for rest of the subjects

then I ran the QCHACHE on the data, made a Qdecrc. file in the subject DIR, and then transformed the longitudinal QDEC table into a QDEC table which looks like:

fsid time gender age Base1 3 female 20 .......... continues for the rest of the subjects.

I then followed the QDEC steps outlined In the group tutorial. (import cross sectional table, generate stats data table, import aseg volume, set longitudinal dependent variables and pick continuous variable.

1) the first problem I am having is that when I go to process the longitudinal group results through QDEC it makes me pick a continuous variable when I just want to analyze the dependent variable The symmetrized percent change. So I have tried picking different continuous variables ( such as age or importing rhcortex volume etc...) with choosing symmetrized percent change as the dependent variable. No matter what continuous variable I pick the question: ``does the average symmetrized percent change differ from zero"? always comes up, the problem is depending on what continuous variable I pick the results change. Is there a way to just look at the longitudinal thickness changes across a group alone, because I first want to look at the atrophy across the whole group over 6 months and then compare the differences in atrophy against the diagnosed and non diagnosed group.

2) another thing I don't quite understand is it seems like the longitudinal qdec tables are made with the cross of the subjects under fsid, but why wouldn't you use the longitudinals under the fsid for the longitudinal table instead? Also it seems like in the cross sectional qdec table the base is the subject being analyzed under fsid, which also doesn't make sense to me because in the tutorial it says base should not be used directly for analysis.

thanks for the help!

Taylor