I'm using SPSS to run a Mixed Model with two categorical (factor) predictors with an interaction between the two predictors. I get the following Estimates of Fixed Effects:
In the interaction I am unclear as to why I get four redundant (reference) categories and only two estimates of fixed effects. I was expecting three reference categories (at most): where all PredictorA =1 combinations of PredictorB would be referenced against PredictorA = 2 combinations. In the output, combinations of PredictorB =10 appears, to me, to have no reference.
Some clarity on how the redundant (reference) parameters work in the interaction would be appreciated.
Here is my syntax:
MIXED DV BY PredictorA PredictorB /CRITERIA=CIN(95) MXITER(100) MXSTEP(10) SCORING(1) SINGULAR(0.000000000001) HCONVERGE(0, ABSOLUTE) LCONVERGE(0, ABSOLUTE) PCONVERGE(0.000001, ABSOLUTE) /FIXED=PredictorA PredictorB PredictorA*PredictorB | SSTYPE(3) /METHOD=REML /PRINT=G SOLUTION TESTCOV /RANDOM=INTERCEPT | SUBJECT(iD) COVTYPE(VC)
Thanks for your time.
Additionally the same happens in this example, taken from UCLA