THE CUPS GAME (Jackson, P., The Cups Game NSF Product Realization Consortium Module Description, Cornell, NY: Cornell University, 1996)

This activity was designed by Peter Jackson . It illustrates the difference between push and pull production. It is also effective in demonstrating the advantages of small lot manufacturing. This activity makes a very convincing argument for just-in-time production and has converted many skeptics.

 

The unit product for the cups game consists of a four-cup holder, containing four cups. The cups have lids and straws and are marked with an adhesive blue dot (see Figure 1). The game requires six participants while the rest of the class are interested spectators. The first participant is the supplier and supplies all raw material to the work stations. The next four participants each work at a workstation. At the first workstation four cups are placed in the cup holder. At the next station dots are placed on the cups. At the third lids are placed on the cups and at the fourth station straws are unwrapped and inserted into the lids. The last participant is the shipping/quality control station.

 

Figure 1. Cups game unit product.

 

This activity is a perfect example that a picture is worth a thousand words. Students can clearly see the difference between the push and pull systems. Many become instant believers. We normally play three iterations of the process each lasting about 15 minutes. The first is push, where each worker produces as much as possible and pushes the work to the next station. In no time, inventories start building up, table space is consumed, and last but not least the process deteriorates into a hectic and chaotic state. Somewhere in the middle of all this, a time piece is introduced to measure the process turnaround time. The unit often gets buried in inventories. The activity also calls for the introduction of a quality problem (red color dots) that often goes unnoticed until quality control at end of the line and numerous red dots later. When the process is stopped, the following evaluation measures are made: WIP, Space, Time in Process and Rework. In the discussion that follows the first iteration students recognize that the excessive WIP at some stations is a problem. Some students suggest that the bottlenecks in the process are the cause of the problem and that we should increase the number of workers at some workstations, add more space, and have quality control at every station. However if we indicate that the current output rate is adequate to meet our demand then students recognize that these suggestions are expensive solutions to the wrong problem.

 

In our second iteration we demonstrate a pull systems. Kanban space (adequate for four units) is defined on the tables using tape for each workstation. Workers are instructed to work a new lot only when their kanban is empty. Students quickly realize that the rate of output is not affected by this change (still determined by the longest task time.) In fact, by eliminating the chaos and clutter workers are likely to work faster. The measures of performance are greatly improved with this iteration. The WIP and defects are limited by the kanbans, the time in process is cut in half, the required table space is down from three to two and the workers are much happier with some making after class plans. Since WIP and reworks are reduced the pull system is actually saving money (with no increase in cost). The most startling point is that by having people work less hard profits are increased. At this point it is easy to see that the WIP can be further reduced by reducing the lot size. The third iteration is pull with a lot size of two. At this point tradeoffs and limitations associated with reducing lot sizes are discussed.

 

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