Yes, specifying subject=subjid or subject=subjid(clinic) would make no statistical difference in your case, but numerically, specifying subject=subjid(clinic) would make the model to be processed by subjects and therefore is more efficient. You must have a common subject in your RANDOM and REPEATED statements in order for the model to be processed by subjects, and that is why nesting here helps -- clicnic is the common subject syntactically.
Thanks,
Jill
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