There are numerous methods of setting priorities in a production environment, too numerous to discuss in total in this paper, but some of the more prevalent methods are discussed below.
First-in, First-out (FIFO)
One means of determining the priority of authorizing work in queue at a given WC is FIFO. By this method, the priority of work is determined chronologically by examining the arrival time of the WOs. The WO that arrived at the earliest time will be the next WO to be authorized for production processing.
Due-Date Slack (AKA Critical Ratio)
Another method of determining the priority of authorizing work in a queue is the difference between the time remaining until its scheduled completion date, the due date, and the time remaining for its Flow Time through the production process. By this method, a WO that is due to complete in five days, for example, and has five days of remaining processing time would have a higher priority than a WO that is due to complete in five days and has three days of remaining processing time. The logic is that the latter WO has two additional days of slack in its production schedule, while the former WO has no slack and must be processed immediately to complete on time.
Another prioritization method is to assign a higher priority to WOs that are being expedited due to management policy. This may happen if the customer of the WO has paid an expedite fee, or the WO is a replacement for a previously delivered WO that failed quality or safety standards. Management may prioritize a customer’s work for a myriad of reasons such as military materials during a time of war. By this method, the expedited WO is always authorized for production ahead of WOs that are not being expedited.
Other Priority-setting Scenarios
There are other settings in which certain products, or customers, may have higher priority than others. These priorities may be assigned numerically to WOs, such as a 1- 10 with 1 being the highest priority and 10 being lowest. In this scenario, WOs for products that are Priority-1 would receive preferential treatment and be authorized ahead of WOs with Priority-5 products.
Regardless of the system for setting priorities, for a given WC, the demand-pull system finds the highest-priority WO in its Upstream-Adjacent WCs and applies its Flow Time to Need Time comparison method to determine whether to authorize the WO for production. If it finds a “tie”, where two or more WOs have identical priorities, it will fall back to FIFO for the subject WOs.