AbstractAn analysis of variance for univariate feedforward/feedback systems has been derived which allows the variance contributions due to both disturbances and controllers to be established. A performance index which assesses the performance of the overall control scheme has been developed. Estimates of the analysis of variance and performance index can be obtained using ordinary closed loop data and least squares regression. These quantities can be used on‐line or off‐line for either the design of new controllers or as a means of assessing the performance of existing feedforward/feedback control schemes. The practicality of the results in the paper are demonstrated using simulated and industrial d