AbstractThis paper describes the use of a novel interface device to display to an operator the behaviour of a computer-simulated plant and to allow him to control the activity of that simulated plant. The human decision-making functions of the operator were studied by analysing the control strategies used and the plans and tactics developed by him in the course of experimental control sessions.The simulated plant used in these experiments was a five-furnace electric-arc steel-melting shop. The operator's task concerned control of electric power to the whole shop to maximize steel output under the restriction of a given energy consumption rate. Undergraduates and steelworks staff familiar with the actual process simulated were used as operators in the experiments.Several notational systems for describing possible and actual control strategies have been studied and the results suggest that one of these provides a feasible method of describing human thought processes during decision-making.