Yavuz and Stair (1995) concluded that reducing the kanban size could reduce the inventory levels and the makespan. On the other hand, increasing the kanban size will increase WIP, but improve the fill rate. Therefore, reducing the kanban size to achieve lower inventory level, and simultaneously retaining the full customer satisfaction (i.e., fill rate) may not be easily implemented in real situations. The kanban size is a vital problem in JIT system which is worthy of being investigated.
Since using the empirical equation to compute the kanban number could result in too many WIPs, many researchers have focused on findings the optimal kanban number in production system. Some researchers tried to find the optimal kanban number based on a cost model.