Simulation-based manufacturing accounting traces cost drivers 'forward' through a computer simulation, measuring (even non-traditional) costs in real time (Son 1990b, 1991a, 1993). For example, whenever a part waits on the manufacturing floor, itswaiting time in each process is accumulated, so that the waiting cost of the part can be estimated by multiplying the cumulative waiting time by opportunity cost (perhaps lost profit) per unit time.