Production Sequencing and Dispatching Rules: How to Decide What Runs Next

Key takeaways

• Dispatching rules decide which job in a queue runs next, and the rule you pick changes which goals you hit.
• Common rules include first come first served, shortest processing time, earliest due date, and critical ratio.
• Each rule optimizes a different objective: flow time, on-time delivery, or fairness.
• No single rule wins everywhere, so the choice should match what the plant is measured on.

When several jobs wait at the same machine, something has to decide the order. Dispatching rules are those decisions, made explicit. They are simple, but the rule you choose quietly determines whether you favor speed, due dates, or fairness.

Why sequencing matters

The set of jobs and their total work is fixed, but the order you run them in changes average wait time, how many orders are late, and how long the longest job sits. Good sequencing extracts more performance from the exact same capacity, which is why dispatching rules matter most at the bottleneck.

Common dispatching rules

• First come first served (FCFS): run in arrival order. Fair and simple, but blind to due dates and job length.
• Shortest processing time (SPT): run the quickest job next. Minimizes average flow time and queue length, but long jobs can starve.
• Earliest due date (EDD): run the most urgent due date next. Good for minimizing lateness.
• Critical ratio: rank by time remaining divided by work remaining, balancing urgency and workload.

A worked example

Four jobs wait at a machine with processing times of 1, 2, 3, and 4 hours. Run them shortest first and the average completion time is far lower than running longest first, because the quick jobs clear the queue early. But if the 4-hour job is due first, EDD would run it now to avoid a late shipment, accepting a higher average wait. Same jobs, different rule, different winner.

Choosing a rule

• Measured on flow time or WIP? Shortest processing time tends to win.
• Measured on on-time delivery? Earliest due date or critical ratio fit better.
• Need simple and transparent? First come first served is easy to explain, if not optimal.

Where OEE fits

Dispatching rules assume each job takes its expected time. When real OEE varies, processing times drift and a rule tuned on paper underperforms on the floor. Sequencing against measured cycle times, especially at the constraint, makes the rules pay off. Book a Fabrico demo to see how live performance data supports smarter sequencing. Dispatching rules also live inside APS and finite-capacity scheduling.

Common mistakes

• One rule everywhere. The best rule depends on the objective; forcing one rule plant-wide leaves performance on the table.
• Optimizing flow time while measured on due dates. Shortest processing time looks efficient but can miss deadlines.
• Sequencing non-bottlenecks. Effort spent ordering jobs at a resource with spare capacity changes little; focus on the constraint.

Frequently asked questions

Which dispatching rule is best?

There is no universal best. Shortest processing time minimizes average flow time; earliest due date minimizes lateness. Pick the rule that matches the metric you are judged on.

Where do dispatching rules matter most?

At the bottleneck. Sequencing the constraint well lifts the whole plant, while resequencing a resource with spare capacity has little effect.