Quantitative Feedback Theory (QFT) and Robust Control

QFT, a theory developed by Horowitz [H3], is claimed by its advocates to provide a complete and general treatment of feedback design for highly uncertain multi-input-output (MIMO) systems. This paper reviews QFT and shows that while the philosophy behind QFT is attractive, the claims for the theory are unjustified. In particular, counterexamples are given for the main theorem of QFT on which the claims are based. This is in spite of the severe assumptions (no rhp zeros and fixed relative degree) that QFT requires on the plant model.