Good morning,

Thanks a lot for this forum, it helped me a lot and is really useful
I have a question because after making an analysis I realized that what I did is probably not statistically correct.

I have a longitudinal analysis (more or less 3 years between my 2 acquisitions T1 and T2). My idea was to compare the change of connectivity between my two acquisitions. However, my subjects have a large range of age so I wanted to add the age as covariate.

My design matrix is the identity matrix on column (size number of subject), a vector of 1 and -1 for the 2 time point and a vector of age:

example for 3 subjects (the 4 first columns: 3 comuns for the subjects one for the time)
1 0 0  1
0 1 0  1
0 0 1  1
1 0 0 -1
0 1 0 -1
0 0 1 -1
My problem and so my question is about the vector of age. Do I use only the age at T1 (so the vector would be [ageT1;ageT1])  because the information is redundant (more or less 3 years apart) or do I make a vector [ageT1;ageT2] considering that the 3 years is not exactly 3years. In the second case, I believe the model adequate should be a model mix linear, can we model it with NBS?

I hope I have been clear in my question/explanation.
Thanks a lot for your help!
Cecile