Dear all,

 

I am trying to perform a longitudinal statistic by using the LME mass univariate toolbox. I have two groups of subjects (each acquired 3 or 4 times over the follow-up), and I want to test whether atrophy progresses differently over time in these two groups.

 

I followed the tutorial on the Website, and I used the following design matrix to test the time x group interaction: X=[ones(length (M),1) M M(:,1).*M(:,2)]

 

I first tried to code the categorical variable “group” assigning values 1 and 2 for subjects belonging to group 1 and 2, respectively (as it is suggested also in the tutorial). Then, I tried to re-run the same model by using mean-centered values for “group” (i.e., assigning -1 to subjects of group 1, and 1 to subjects of group 2).

 

When I run the two statistical models, I obtain different results. Therefore, I would like to ask which is the correct way to model the group covariate? Should it be zero-centered or not? How de-meaning is affecting the estimate?

 

Thank you so much for any suggestion

Kind regards

Paola