1.  The choice of GLM or MIXED depends on the error structure, and not the fact that you want have a fixed effect model or you want to test a linear combination. GLM assume the errors a iid normally distributed. MIXED allows for errors to be non-iid errors.   2. I believe you would use the ESTIMATE statement in GLM (and I think in MIXED) to obtain the estimate for your linear combination. Since you have no CLASS variables, you could also use PROC REG and the TEST command. ... View more