Denominator degrees of freedom for fixed effects in SPSS MIXED
I've found that a mixed linear model run in the SPSS procedure MIXED reports denominator degrees of freedom (df) for the fixed effects in the model that differ from those reported by the SAS procedure Proc Mixed. Will you explain the difference in results between the two programs?
Resolving the problem
SPSS MIXED uses the Satterthwaite approximation to calculate df. This method is valid for both balanced and unbalanced designs. By default, SAS uses the CONTAINment method when there is a RANDOM statement, and the BETWITHIN method when a REPEATED statement is used without a RANDOM statement (unless the R matrix is specified as unstructured). These methods do not adjust for lack of balance in the data. Neither does the RESIDUAL, method available in SAS. SAS does offer two methods that adjust for lack of balance: These are the SATTERTHwaite and the KENWARDROGER methods. While the CONTAIN, BETWITHIN, and RESIDUAL methods always yield integer values for the denominator df, the SATTERTHwaite and KENWARDROGER do not.
If you want to compare SAS results for mixed linear models to those from SPSS, be sure you have employed the Satterthwaite method for computing denominator degrees of freedom in SAS.