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Dear mailinglist, Actually, I am working on a study where two groups of subjects were assessed across multiple study sites. My question concerns the ratio of recruited subjects of each site (or in other words, of each scanner) separately. Does anybody know if it is problematic to include for example groups with a 2:1 ratio (i.e. 20 controls, 10 patients) at site 1 and 2:1 ratio at site 2? I included study site as a covariate of no-interest in the statistical fMRI model but, nonetheless, I am wondering if there are any compelling reasons speaking against the analysis of the abovementioned unbalanced groups across sites. I would be very grateful for any help (or references) in this regard. thanks a lot Chris