Use this option to specify the denominator degrees of freedom (DDF) method to be used when random effects are included in the model .Note : This option is enabled only when a random effect is specified on the Model Variables tab.The option selected is used for the DDFM= option in the MODEL statement of the MIXED or GLIMMIX procedure. Refer to the SAS PROC MIXED documentation or SAS PROC GLIMMIX documentation for additional details.
The DDF are computed by dividing the residual degrees of freedom into between-subject and within-subject portions. PROC MIXED then checks whether a fixed effect changes within any subject. If so, it assigns within-subject degrees of freedom to the effect. Otherwise, it assigns the between-subject degrees of freedom to the effect. If there are multiple within-subject effects containing classification variables , the within-subject degrees of freedom are partitioned into components corresponding to the subject-by-effect interactions. Performs all tests by using the residual degrees of freedom, n-rank(X) , where n is the number of observations . Performs the degrees of freedom calculations by inflating the estimated variance - covariance matrix of the fixed and random effects. Satterthwaite-type degrees of freedom are then computed based on this adjustment. By default, the observed information matrix of the covariance parameter estimates is used in the calculations. For covariance structures that have nonzero second derivatives with respect to the covariance parameters, the Kenward-Roger covariance matrix adjustment includes a second-order term. This term can result in standard error shrinkage. Also, the resulting adjusted covariance matrix can then be indefinite and is not invariant under reparameterization.