10.2: Points Made in the Case Study

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Previously, a process model has represented the physical movement of items through a system. In this application study, the model must represent both the flow of control information that specifies where and when to route items as well as the movement of those items through a system.

Often, models evolve from previously existing models. The model of the job shop with a push system orientation presented in chapter 8 is evolved into a model of the job shop with a pull orientation, including inventory management.

There is a trade-off between maximizing the service level to customers and minimizing the inventory on hand. The service level is the percent of demands that are met on time. A series of simulation experiments can be run varying the number of kanbans, and thus inventory capacity, of each type of product to help quantify this trade-off.

In push systems, inventory level is a consequence of how the system operates and can be a simulation experiment performance measure. In a pull system, the inventory level is a model parameter whose value is to be set through simulation experiments.

A high service level to customers may be achieved even though work-in-process inventory is not always available when a workstation is directed to perform its operation. In other words, a workstation may be starved. Thus, a high service level to each workstation may not be necessary.

Previous information about how a system operates, or should operate, can be combined with simulation results to draw conclusions. This technique is known as the use of prior information. In this case, the expected mix of item types demanded is known and is used in conjunction with performance measure estimates to set the number of kanbans. In addition, the number of kanbans for an item type could be the same in all inventories to minimize the number of inventory parameters. This number will be based on the number needed to provide the required customer service level in the finished goods inventory.