Reference Work Entry

Encyclopedia of Production and Manufacturing Management

pp 324-324

JOB SEQUENCING RULES

Abstract

In the last two decades, production and manufacturing management has rapidly adopted a range of new concepts: manufacturing strategy, focused factory, just-in-time manufacturing, concurrent engineering, total quality management, supply chain management, flexible manufacturing systems, lean production, mass customization, and more. The increasing globalization of manufacturing ensures that the field will continue to expand. The Encyclopedia of Production and Manufacturing Management will be useful to a technically diverse audience of managers, engineers and process consultants concerned with manufacturing techniques, methods, and manufacturing decisions. With 100 in-depth articles, and more than 1,000 shorter entries, and a continual flow of revisions and new material, the Encyclopedia will help experts, educators and students keep up with the leading edge of innovation.

Abstract

In the last two decades, production and manufacturing management has rapidly adopted a range of new concepts: manufacturing strategy, focused factory, just-in-time manufacturing, concurrent engineering, total quality management, supply chain management, flexible manufacturing systems, lean production, mass customization, and more. The increasing globalization of manufacturing ensures that the field will continue to expand. The Encyclopedia of Production and Manufacturing Management will be useful to a technically diverse audience of managers, engineers and process consultants concerned with manufacturing techniques, methods, and manufacturing decisions. With 100 in-depth articles, and more than 1,000 shorter entries, and a continual flow of revisions and new material, the Encyclopedia will help experts, educators and students keep up with the leading edge of innovation.

In a job shop, it is common for several jobs to wait in a queue for processing at each work center. The order in which the waiting jobs are processed can have a profound effect on the number of jobs waiting in the plant, the number of jobs that are delivered on-time, the time the jobs spend in the plant, the work-in-process inventory, and the average lateness of deliveries to customers. Therefore, the sequencing of jobs at work centers in an important decision. Since thousands of sequences must be determined every week in an average plant, it is important to develop simple and practical rules for job sequencing.

Job sequencing rules, or dispatching rules are used for sequencing jobs at a work center. A few of the common rules are:
  • Earliest due date rule (EDD) – job that is due first is started first. Used when companies are sensitive to due date changes.

  • Shortest processing time rule (SPT) – this gets the most work done rapidly and minimizes work-in-process (WIP). A drawback of this rule is that jobs with very long processing time may get delayed. To prevent this, periodically the rule is changed to longest processing time rule.

  • Longest processing time rule (LPT) – this rule increases the WIP and may cause many short jobs to miss their due dates.

  • First come, first served rule (FCFS) – it is the fairest rule, especially when people are to be sequenced at a service facility. It does not perform as well as the SPT rule.

  • Critical ratio rule (CR) – this rule combines the due date and processing time. CR is computed by the ratio:
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In selecting a rule for use, one or more performance criteria are employed. A few of the common performance criteria used in evaluating sequencing rules are:
  • Average time to produce the item (called average flow time, which includes waiting and processing time).

  • Average WIP inventory.

  • Average lateness of jobs (a job is late if it is completed after a known due date).

Often, a simulation can be performed to assess the most appropriate rule to use in offer to meet one or more of the performance criteria specified by management.

See Manufacturing systems; Setup reduction; Schedule stability; Simulation analysis of manufacturing logistics systems; Simulation of production problems using spreadsheet programs.

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© Kluwer Academic Publishers 2000
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