Hi JacobSimonsen, Thank you for your reply. I think you are right. When using vents/trials syntax, SAS automatically apply DDFM=containment option, which leads to df=14. But in my program, DDFM=BETWITHIN should be used, since it accounts for correlation within clusters. I found a paper disccusing this question "Comparing denominator degrees of freedom approximations for the generalized linear mixed model in analyzing binary outcome in small sample cluster-randomized trials". It suggests that "Between-Within denominator degrees of freedom approximation method for F tests should be recommended when the GLMM is used in analysing CRTs with binary outcomes and few heterogeneous clusters, due to its type I error properties and relatively higher power".
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