What are Priority Rules? Definition & Manufacturing Examples

What are Priority Rules?

Priority rules are the decision logic used to determine the order in which jobs are selected for processing when multiple jobs compete for the same resource. Also called dispatching rules or sequencing rules, they provide a systematic, repeatable method for making sequencing decisions. Priority rules eliminate the subjectivity and inconsistency that occur when operators or supervisors choose jobs based on intuition, familiarity, or personal preference.

How Priority Rules Work in Manufacturing

When a machine becomes available and several jobs are waiting in queue, the priority rule determines which job runs next. The most common priority rules include:

Earliest Due Date (EDD): The job with the soonest due date goes first. This rule minimizes the maximum lateness of any single job and is the most intuitive rule for manufacturers focused on on-time delivery.

Shortest Processing Time (SPT): The job with the shortest operation time goes first. This rule minimizes average flow time and average number of jobs in the system. It clears the queue quickly but can leave long jobs waiting indefinitely.

First Come First Served (FCFS): Jobs are processed in the order they arrive at the work center. Simple and perceived as fair, but often produces poor due date performance.

Critical Ratio (CR): Calculated as time remaining until due date divided by processing time remaining. A ratio below 1.0 means the job is behind schedule. Lower ratios get higher priority. This rule dynamically adjusts priorities as conditions change.

Minimum Setup Time: The job requiring the least changeover from the current machine setup goes first. This maximizes machine utilization but may hurt due date performance if frequently overriding due date priorities.

Priority Rules Example

Five jobs are waiting at a CNC mill. The machine just finished its current job and needs the next assignment:

• EDD sequence: A, C, E, B, D — focuses on meeting nearest due dates
• SPT sequence: E, C, A, D, B — clears short jobs fast, reducing average wait time
• FCFS sequence: B, D, A, E, C — fair but ignores urgency
• CR sequence: A, C, E, B, D — prioritizes jobs falling behind schedule

The EDD and CR sequences are identical here because the jobs closest to their due dates also have the lowest critical ratios. SPT would finish 4 of 5 jobs faster on average but might make Job B late.

Why Priority Rules Matter for Production Scheduling

Priority rules are the engine behind automated scheduling. Every time a scheduling system determines which job runs next on a machine, it applies one or more priority rules. The choice of rules directly affects on-time delivery, average lead time, WIP levels, and machine utilization.

Resource Manager DB (RMDB) allows planners to configure priority rules that match their business objectives. A shop focused on on-time delivery might use EDD as the primary rule with setup minimization as a tiebreaker. A shop focused on throughput might use SPT with due date constraints. The flexibility to tailor and combine rules is a key advantage of modern scheduling software.

Related Terms

• Job Sequencing — The process of ordering jobs at a work center, driven by priority rules
• Dispatch List — The ordered list of jobs at each work center that reflects priority rule decisions
• Slack Time — A scheduling metric used by some priority rules to assess job urgency

Frequently Asked Questions

Learn more in our complete manufacturing glossary or production scheduling guide.