Non-Convex Costs and Capital Utilization: A Study of Production and Inventories at Automobile Assembly Plants

This paper studies how managers at automobile assembly plants organize production across time. Detailed data from eleven single-source automobile assembly plants display considerable cross-plant heterogeneity. At plants which make low- and medium-selling vehicles the capital stock often sits idle, production is more variable than sales, and weeklong shutdowns are often used to vary output. In contrast, at plants which make high-selling vehicles, the capital stock rarely sits idle, production is about as variable as sales, and overtime, not weeklong shutdowns, is most frequently used to vary output. To explain this difference in production scheduling, I formulate and solve a dynamic programming model of a plant manager. The solution to the dynamic program predicts that when sales are low, non-convexities at the plant level induce the manager to bunch production at points of low average cost; thus, the manager uses less than full capital utilization on average and makes production more volatile than sales. When sales are high, the plant operates in a convex region of the cost curve. Hence the manager employs high levels of capital utilization and makes production about as volatile.