Dear Alfonso,

I have specified my three sites as three covariates: site 1, 2 and 3 with values 0 and 1. I want to look at the effect of AllSubjects. If I include all three of them [1 0 0 0], I get the warning: 'possibly incorrect model: non-estimable contrasts (suggestion: simplify second-level model). As I understood from your answers in this thread, it should not matter when I leave site 1, 2 or 3 out. However, I do get different results when I leave one of them out, and it also depends on which one I leave out.. Can you help me understand what is going wrong here?

Best regards,
Nikki