Dear Experts,
     I have data from a mixed design with 2 within subject factors (A, B each with 2 levels) and a between subject factor (Group, 2 levels). Unfortunately, it ended up that the number of subjects is different in the 2 groups. My understanding is that when the variance components are estimated using ReML, it should work irrespective of whether the design is balanced or not (right?). However I have the following questions:

If I specifiy A,B and Group as fixed effects (with subject as the only random effect):

(1) How does SPM know how to calculate the F ratio for the main effect of Group? Should I specify what the appropriate error term is?
(2) Given that all factors have only 2 levels, do I still need a sphericity correction? Also, is there a way to specifiy the model differently to circumvent dealing with sphericity assumptions?
(3) I have in the past read especially in SPLUS forums about issues with determining the 'denominator' degree of freedom when using ReML.. Any references on how to understand that would be great..

Any comments and references are most appreciated.