The single loop control system with controller of general structure or with PI/PID structure is studied. The control task of the system is to track step set-point changes or reject the intermittent step output disturbances. Performance of a simple feedback system is assessed with its IAE value (J) and its rise time (t(r)) observed from the response of the system to a step set-point change. Assume that the dynamics of open loop processes can be represented by models of first order or second order plus dead time (i.e., FOPDT or SOPDT). On the basis of these models, the optimal IAEs and the associated rise times are computed. The performance of a system is assessed by comparing its current IAE to the optimal IAEs. An index is thus defined for this quantitative assessment. To be free of a process model for assessment, envelopes of optimal IAE and the rise time are prepared. A method to estimate the step response for set-point tracking by making use of the response of the system to dither inputs is presented. By introduction of dither inputs intermittently to the system, the performance of the single loop can be monitored on-line.