Start with something familiar
At a busy desk you can answer the shortest notes first or start with the note that needs a reply soonest. The short-first approach clears many items quickly. The due-first approach protects the nearest promise. Neither label tells you everything about the result. If one long reply is due almost at once, the two choices can feel very different to the person waiting. A factory queue has this same conflict in a form we can measure.
Words to know: EDD, SPT, Completion time, Tardiness.
State the narrow experiment
For this experiment, one machine is always available, all jobs are released at zero, and each job runs without interruption. There are no setups or unequal job weights. These conditions let us isolate the effect of sequence. Add a release delay or family change later, and you must calculate again rather than carry a conclusion across unchanged.
SPT orders the jobs by processing duration. EDD orders them by due time. Break ties alphabetically here so the comparison can be reproduced. Both rules produce a sequence, not an explanation of what the business values. State the measure first: average completion, worst lateness, number late, and total tardiness are different questions.
Calculate before choosing
For each job, add its processing duration to the running clock. Subtract its due time to find lateness, then replace negative values with zero to obtain tardiness. Keep negative lateness in its own column; early work can matter for storage, but it must not cancel a late job when you report total tardiness.
Under the narrow all-ready single-machine assumptions, SPT minimizes total completion time and EDD minimizes maximum lateness. These are limited results, not claims that either rule wins every service measure. This course uses the original Northstar calculation to make the distinction visible without requiring a proof or a solver.
Make a comparison that a colleague can challenge
Record all job completions rather than only a single score. A rule may improve the average by making one customer wait much longer. A second rule may protect an urgent order but delay several small jobs. The right decision requires an explicit service policy; arithmetic alone cannot choose the importance of those promises.
Test a changed due time without changing processing times. SPT order remains the same because its input did not change. EDD may change because its ranking input did. This simple experiment helps identify what each rule can and cannot respond to. Keep the original result visible so a reader sees the effect of only one changed fact.
- Validate inputs
- Place legal work
- Check every resource
- Compare outcomes
- Release with conditions
Northstar worked schedule
All data are fictional. Time is elapsed minutes from the stated shift origin; unlisted transfer times are zero. Use the explicit resource and setup assumptions below.
| Job | Processing minutes | Due minutes |
|---|---|---|
| A | 60 | 70 |
| B | 20 | 120 |
| C | 40 | 100 |
- 1. EDD gives A-C-B with completions 60, 100, 120.
- 2. SPT gives B-C-A with completions 20, 60, 120.
- 3. EDD total completion is 280; SPT total completion is 200. EDD total tardiness is 0; SPT total tardiness is 50.
EDD mean completion is 93.33 minutes; SPT mean is 66.67. Both makespans are 120; only EDD meets every due date.
Assumptions
- All times and quantities are deterministic inputs unless explicitly labeled expected or scenario-dependent.
- Operations cannot be split; touching interval boundaries are allowed. Additional constraints are only those stated in the lesson.
Where this can fail: These small cases teach traceable decisions, not a promise of performance in an unmodeled plant.
Build, test and explain a plan
Use A 60 due 70, B 20 due 120, C 40 due 100, all released 0, one machine available 0-240, no setups. Build all six permutations. Compare mean completion, maximum lateness, total tardiness, and number tardy. Then change A due to 150, and separately change B duration to 50 with original dues. Recalculate both rules in each scenario. The eight hours are self-paced estimates for modeling, scenario work, checks, and explanation; they have not been experimentally validated.
Use paper or a spreadsheet. The estimates below include the study and worked example already on this page.
| Estimated hours | Activity |
|---|---|
| 0.5 | Study the lesson and make a one-page input dictionary. Define the clock, units, resource names, release rule, and all assumptions. Mark which inputs are fixed and which will change in the scenarios. |
| 0.5 | Reproduce the worked example without looking at its result, then compare every intermediate start and finish. Explain any difference in a short error log before building the larger table. |
| 3 | Build a complete operation-by-operation planning table for every requested sequence or scenario in the lab brief. Include starts, finishes, blocked intervals, due comparisons, and a resource timeline. Cross-check each row against its input and reconcile resource totals. Use paper or a spreadsheet; no special software is required. |
| 2 | Run the specified stress cases independently, restore the base between cases, and compare each changed result with the original. Create a small sensitivity table and identify which assumption caused each difference. Audit feasibility before ranking the outcomes. |
| 1 | Write a decision memo with the selected feasible option, numeric evidence, the losing option’s advantage, and any unmet promise. State the condition that would cause you to reconsider. Do not invent additional capacity or missing data. |
| 1 | Teach the result aloud or in writing to an imagined operator. Rebuild one key check from a blank page, answer the recap questions, and revise the memo if your explanation reveals an unsupported assumption. |
Your deliverables
- Input dictionary and checked schedule table.
- Scenario comparison with job-level or quantity-level evidence.
- Decision memo and teach-back record.
Check your practice work
- Baseline EDD lateness is A -10, C 0, B 0; maximum 0.
- Baseline SPT lateness is B -100, C -40, A 50; total tardiness 50.
- With A due 150, EDD C-B-A completes 40,60,120 with no tardiness; SPT also has none.
- With B duration 50, SPT C-B-A completes 40,90,150; A tardiness 80. EDD A-C-B completes 60,100,150; B tardiness 30.
Keep these three ideas
- Compute EDD and SPT outcomes without treating a rule as a universal answer.
- Which constraint or metric changes the choice in this case?
- What assumption must be confirmed before this plan can be released?
Common mistakes
- Comparing plans before checking every stated constraint.
- Changing several assumptions at once and losing the explanation for the result.
Optional video study
Compute EDD and SPT outcomes without treating a rule as a universal answer.
- Introduce the specific Northstar inputs
- 1. EDD gives A-C-B with completions 60, 100, 120.
- EDD mean completion is 93.33 minutes; SPT mean is 66.67. Both makespans are 120; only EDD meets every due date.
First half of module narration
Open the module media & quizPut this idea to use
Carry your assumptions into a practical tool, then compare your result with a short guide.
Open a related planning toolRead the related planning guidePractice with the planning gameThis saves a completion marker in this browser only. It does not verify time spent.