You use the CLASS and MODEL statements to define a linear model for the runs in the candidate data set. You can also use these statements to define a general covariate model. In this case, list the CLASS and MODEL statements that define the model for the candidate points directly after the PROC OPTEX statement. Then list the CLASS and MODEL statements that define the covariate model after the BLOCKS DESIGN= specification. Thus, in this case, the ordering for these statements should be

Note also that a CLASS statement naming classification variables must precede the MODEL statement that uses those variables.