For cascade control Havre (1998) has shown how to select secondary measurements such that the need for updating the setpoints is small. The issues here are similar to that of selecting controlled variables (c) discussed above. One approach is to minimize some norm of the transfer function from the disturbance and control error in the secondary variable to the control error in the primary variable. A simpler, but less accurate, alternative is to maximize the minimum singular value in the transfer function from secondary measurements to the input used to control the secondary measurements. Lee and Morari ((Lee and Morari, 1991), (Lee et al., 1995) and (Lee et al., 1997)) considered a similar problem. They used a more rigorous approach where model uncertainty is explicitly considered and the structured singular value is used as a tool.