Start with something familiar
When a road closes during a delivery trip, the stops already made stay made. You reroute the remaining stops from the vehicle’s actual location, rather than pretending the trip started again. A production disruption needs the same discipline. Completed jobs are facts, active work needs a clear rule, and future work can be moved. Keeping those categories separate makes the new plan both honest and useful to the people carrying it out.
Words to know: Frozen horizon, Release time, Dispatch, Calendar.
Start at the event clock
At minute sixty, Northstar learns that the filler will be unavailable until minute one hundred. Job A has just completed, so it is history. Future B and C must be repaired from minute one hundred. Record the event, its effective time, and the assumed restoration time. Without these facts, two repair plans may be solving different problems.
This case places the breakdown exactly between jobs, so no interrupted batch needs a quality or restart decision. That removes a common ambiguity. If a later case interrupts active production, you must explicitly define whether it resumes, restarts, or is scrapped. Never assume remaining duration equals original duration minus elapsed time without checking the rule.
Separate legal from preferred
A frozen horizon protects already communicated work near the present. In the Northstar policy, starts promised before minute one hundred twenty are protected from routine changes. A breakdown can make those starts physically impossible. The response is a documented exception, not a false promise that frozen work can ignore machine availability.
First build a feasible repair that preserves order. Then test a small alternative such as swapping the two remaining jobs. Compare service and movement from the baseline. A repair that finishes sooner may be irrelevant if total remaining duration is fixed; the real gain may be a lower worst delay or a better protected customer.
Publish an actionable difference
A repair notice should state old and new starts, affected due times, and the next review trigger. Keep the old version so people can tell which instruction was replaced. Assign one owner to release the new sequence. Multiple unofficial changes can create more disruption than the original equipment event.
The lab uses a second restoration time to expose uncertainty. A planned repair is conditional on that assumption. Give the supervisor a fallback trigger rather than a confident single finish that ignores recovery risk. This is an original teaching workflow, not a claim that every organization uses the same frozen horizon or escalation rule.
- 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 | Original start | Run minutes | Due |
|---|---|---|---|
| A | 0 | 60 | 70 |
| B | 60 | 40 | 110 |
| C | 100 | 50 | 170 |
- 1. A is complete at60. Machine outage occupies60-100.
- 2. Preserve order: B100-140 and C140-190.
- 3. B tardiness30 and C tardiness20; both starts move40 minutes.
Preserve-order repair finishes190; remaining total tardiness50; absolute start movement80 minutes.
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 the example, all released0, one machine open0-300 except outage60-100, no setups. A is completed; freeze originally promised starts before120. Compare B-C with C-B, allowing documented outage exceptions. Recalculate if restoration is130. Separately add urgent D released100, run20, due135; compare B-D-C, D-B-C, and C-D-B after the original outage. 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
- C-B after outage: C100-150, B150-190; total tardiness80, absolute movement90.
- Restore130, preserve order: B130-170,C170-220; tardiness60+50=110.
- With urgent D, B-D-C gives B140,D160,C210; tardiness30+25+40=95.
- D-B-C gives D120,B160,C210; tardiness0+50+40=90. C-D-B gives C150,D170,B210; total135.
Keep these three ideas
- Rebuild remaining work from a factual event without rewriting completed history.
- 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
Rebuild remaining work from a factual event without rewriting completed history.
- Introduce the specific Northstar inputs
- 1. A is complete at60. Machine outage occupies60-100.
- Preserve-order repair finishes190; remaining total tardiness50; absolute start movement80 minutes.
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.