Dear Salem,

> The whole discussion of fixed effects and random effects are best left
> to the field of statistician. If you want the details of why it could
> be considered either, see Ch.12 in Human Brain Function.

To follow on what Donald said, a couple of further reference that may be of interest regarding mixed effects and group studies:

Friston KJ, Stephan KE, Lund TE, Morcom A, Kiebel S (2005) Mixed-effects and fMRI studies. NeuroImage 24:244-252.

Mumford JA, Nichols T (2009) Simple group fMRI modeling and inference. NeuroImage 47:1469-1475.

Best regards,

Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104