Method of testing a system model

Testing a system model, the system model comprising a system parameter and three or more equipment models, wherein each equipment model comprises an equipment parameter which models a parameter of a respective equipment. The testing comprises: 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; operating the system model to determine a series of test values of the system parameter, wherein each test value of the system parameter is determined in accordance with a respective one of the test values of the equipment parameter; determining a weight of the selected one of the equipment models, wherein 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 outputting the weight of the one of the equipment models.