Start with something familiar
A washed shirt cannot be folded before it is dry. A dryer and a folding table can serve different loads at the same time, but each load still follows its own order. If the dryer is unavailable for a short repair, a long load may have to wait even when the room looks quiet. Factory routes work the same way: the order within a job and the availability of each resource must both be respected.
Words to know: Precedence, No overlap, Calendar.
Two different kinds of conflict
A route links steps within one job. Northstar fills bottles and then packs them. Filling A must finish before packing A starts, even if the packer is empty. Resource competition links different jobs: A and B cannot both occupy the only filler. A valid plan needs both checks; passing one does not imply passing the other.
Treat each resource as its own timeline. Put the job name on every interval and show unavailable windows as occupied blocks. A route arrow connects the end of a filling interval to the start of its packing interval. The drawing is useful because it exposes mistakes that totals by day can hide.
An interruption rule changes the answer
In this lesson operations cannot pause once started. If a seventy-minute operation would cross a closure, it must wait for a long enough open window. Do not quietly stretch the operation across a break while also claiming it is uninterrupted. A different production process might permit pauses, but that would be a different explicit model.
An operation can wait between steps without holding the first machine. This case assumes unlimited intermediate space and immediate transfer. Those assumptions make the exercise manageable; they do not describe every plant. If a filled bottle must be packed within ten minutes, that extra rule would need its own check and might change the sequence.
Finish the route, not just the first step
Customer completion here means packed completion. A job whose filling ends on time can still miss its due date while waiting for the packer. Compare due times with the final operation only. Show both resource finishes so another reviewer can understand where waiting occurs and whether a proposed improvement actually helps.
Use a small feasibility audit before discussing speed: every operation appears once, its duration is correct, its start meets its predecessor, and its interval stays inside an open resource window. Then inspect shared-resource intervals in time order. This audit can be performed with paper strips, a spreadsheet, or code; none requires an optimization package.
- 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 | Fill minutes | Pack minutes | Due |
|---|---|---|---|
| A | 60 | 30 | 130 |
| B | 70 | 40 | 260 |
- 1. Filler is open 0-120 and 150-300; packer is open 0-300.
- 2. Fill A 0-60 and pack A 60-90. B cannot fit into filler gap 60-120.
- 3. Fill B 150-220 and pack B 220-260.
A completes at 90 and B at 260; the final finish is 260 and neither job is tardy.
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 table, both jobs released at 0, and the calendars in the example. Compare fill orders A-B and B-A. Packing follows first available completed fill. Rebuild both orders when filler closure becomes 120-180. In a separate scenario restore closure 120-150 and make B fill duration 50. Keep all other data fixed. 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
- B-A baseline: fill B 0-70, pack B 70-110; fill A 150-210, pack A 210-240. Last finish 240, but A is 110 minutes tardy.
- A-B with closure ending 180: B fill 180-250 and pack 250-290; B tardiness 30.
- B-A with closure ending 180: A completes 270 and is 140 minutes tardy.
- With B fill 50, A-B fills A 0-60 then B 60-110; packs A 60-90 then B 110-150. Last finish 150.
Keep these three ideas
- Build a two-resource schedule that survives precedence and downtime checks.
- 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
Build a two-resource schedule that survives precedence and downtime checks.
- Introduce the specific Northstar inputs
- 1. Filler is open 0-120 and 150-300; packer is open 0-300.
- A completes at 90 and B at 260; the final finish is 260 and neither job is tardy.
Second 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.