Some graphical sensitivity reduction, tests for time-invariant linear and non-linear feedback systems are presented. These tests are useful in a number of engineering problems where one needs to analyse the sensitivity behaviour of a multivariate plant which has been modified by the introduction of a 1-input-l-output feedback compensator. They are based on a Nyquist. plot and describing-function techniques and involve little extra effort in addition to that which is usually required to determine stability. The modalities of application are particularly well suited for the analysis of power systems control loops. They are illustrated via the analysis of the effect of introducing a frequency-voltage intercoupling in the control system of an isolated hydroelectric power plant.