Priority rule (production)
According to REFA, priority rules are agreements on the order in which several tasks or partial tasks are carried out by a work system according to their urgency . A priority rule in production planning is a simple heuristic for machine occupancy planning and thus a rule for sequence planning of orders to be processed.
Priority rules are also required if the planned completion dates are the same. Following such rules does not lead to an optimal solution to the problem, but it prevents personal interests and power possibilities of individual employees and managers in a production from taking the place of systematics of any kind.
Typical priority rules are First-Come, First-Served (FCFS) or Shortest Operation Time (KOZ, Shortest-Job-Next ). Other common rules are listed in the Elementary Priority Rules screen . The field “local reference” in the picture is set if the rule can be applied in such a way that only the immediate situation in the work system is taken into account. In the case of “comprehensive”, data is used that basically refer to the situation of the entire order and not the situation directly on the work system. The distinction is important insofar as only rules with a local reference can easily be used in decentralized production control processes . If both references are possible, the possible interpretations of the basically same rule are different.
In an older simulation study by Conway et al. KOZ was found to be the generally most favorable rule. Of the rules they tested, this one achieved the lowest expected value for the throughput times that occurred . In many cases this result was combined with the lowest standard deviation of the lead times. However, the rule became more and more unfavorable, the wider the spectrum of the pure order times became. In order to give preference to KOZ, a fairly homogeneous structure in the order times is required.
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- ↑ See Conway, R .; Jonson, B .; Maxwell, W .: An Experimental Investigation of Priority Dispatching. In: Journal of Industrial Engineering 11 (1960), p. 221 ff.