proc model data=compart; endogenous conc; exogenous time; parms a1 95.0 a2 120.0 theta1 1.5 theta2 0.12; conc=(a1*exp(-theta1*time))+(a2*exp(-theta2*time)); resid.conc=resid.conc/(sqrt(pred.conc)); fit conc; run; quit;