A method of testing a system model is disclosed. The system model includes a system parameter and three or more equipment models, each equipment model includes an equipment parameter which models a parameter of a respective equipment. The method includes (a) making a series of changes of the equipment parameter of one of the equipment models so that the equipment parameter of the one of the equipment models adopts a series of test values of the equipment parameter; (b) operating the system model to determine a series of test values of the system parameter, each test value of the system parameter is determined in accordance with a respective one of the test values of the equipment parameter; (c) determining a weight of the selected one of the equipment models, the weight of the one of the equipment models indicates a sensitivity of the system parameter to the series of changes in the equipment parameter of the one of the equipment models; and (d) outputting the weight of the one of the equipment models.