A tuning method used to select optimum controller settings. It is used for the control of complex systems where little is known of the dynamics of the system or where calculation of the response is too difficult or impractical. The method was developed by American control engineers John Ziegler and Nathaniel Nichols and is based on empirical tests that gave good performance for a step response of an open loop system. It is based on the assumption that an open loop transfer function can be approximated by a first-order system with a time delay. The recommended controller settings have been proposed for proportional, proportional and integral, and proportional, integral, and derivative action control. Applied to both open and closed loop systems, the settings are designed to give underdamped transient responses with a decay ratio of a quarter.