A model of a particular type of conveyor-based single stage working has been developed and programmed for a digital computer. The model has been used to simulate “mechanical effect” (i.e. the effect of specific settings of feed rate and tolerance time) on an operator's natural unpaced performance. It is shown that operator response to pacing can be dimensioned more properly by reference to “mechanical effect” measures. Also, with assumptions of operator response to pacing the model can be used to predict output, misses and delay for selected conveyor settings.