Dear CONN Experts,

I would like to evaluate a group*condition interaction (2 groups with 2 exams each) with a second-level covariate. I believe it would be an example of a mixed 2x2 ANCOVA design (subject effects [1 -1 0] with between-condition contrast [1 -1]). Actually, the covariate is not a nuisance covariate but rather a change in behavioral scores between the sessions. Hence, I am more interested in the interaction covariate*condition, i.e., with subject effects set to [0 0 1]. However, I am concerned that the covariate (change in a score) is not really meaningful if conditions are modelled separately.

One solution would be to somehow include a session-specific covariate (i.e., original scores per session). I could not find such design description in the documentation and I searched the forum to find a several related posts still waiting for reply:

https://www.nitrc.org/forum/message.php?msg_id=38860
https://www.nitrc.org/forum/message.php?msg_id=40691
https://www.nitrc.org/forum/message.php?msg_id=41198

A generative AI suggested to include the session-specific covariate as a first-level covariate set to a constant value per subject. Unfortunately, I found no such recommendation here.

Thanks in advance!

Kind regards

Pavel