Start with something familiar
Think of making lunch for a small group with one oven. A list of meals does not tell you when each tray enters the oven. Some ingredients arrive later, and the oven needs time to warm. You need names, durations, and ready times before you can make a useful order. A kitchen clock also matters: twelve minutes after noon is not the same thing as twelve minutes after the delivery arrives. A factory work list has the same need for a shared clock.
Words to know: Job, Operation, Release time, Calendar.
A job list is an agreement
Northstar sells reusable bottles. A customer request becomes schedulable only when someone identifies its quantity, route, input readiness, due time, and allowed machine. Use one row for each operation. Keep job and operation names distinct so a packing step cannot be mistaken for a second order. Record minutes from shift start rather than mixing wall-clock text and elapsed time.
The person reading the list should know whether a quantity means starts or accepted bottles. Ten starts may not produce ten accepted bottles. Durations in this lesson are supplied job times, so do not multiply them by quantity again. A clear units column prevents an error that can otherwise look like a capacity crisis.
Readiness is more than priority
A job can be urgent and still be impossible to start. Release time marks when the required inputs are available under the case assumptions. An earliest due date rule must choose only from jobs that are ready. A planner who ignores readiness creates a neat but unusable queue. Keep a separate blocked list with a reason and owner.
Use half-open intervals: a job running from zero to sixty occupies the machine until sixty, and another may start exactly at sixty. This convention removes false conflicts at touching boundaries. The schedule should include idle intervals too. An idle gap can reveal a missing input rather than weak effort by the operator.
Check rows before optimizing
Start with a legal sequence and calculate each finish as start plus duration. The next start must be at least both the previous finish on that machine and the job release. This small calculation is a transparent baseline. Keep the raw inputs beside it so every result can be traced without relying on a hidden assumption.
A practical release record also identifies the input source and the time it was last checked. In this fictional case all releases are firm. In real work that assertion would need an owner. The lesson distinguishes a schedule built from facts from one built from wishes; it does not ask you to collect real company records.
- 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 | Minutes | Release | Due |
|---|---|---|---|
| A | 60 | 0 | 90 |
| B | 40 | 80 | 150 |
| C | 30 | 0 | 170 |
- 1. For the given A-B-C sequence, A starts at 0 and ends at 60.
- 2. B cannot start before 80, so show idle time 60-80; B ends at 120.
- 3. C runs 120-150; all three supplied due times are met.
A-B-C has 130 busy minutes, 20 idle minutes, and a last finish of 150.
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 one filler available 0-240, no setups, and jobs A=(60, release 0, due 90), B=(40, release 80, due 150), C=(30, release 0, due 170). Build all six sequences with earliest feasible starts. Then move B release to 120 and separately extend A duration to 90. No splitting or overtime is allowed. 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 A-C-B: A 0-60, C 60-90, B 90-130; no idle and last finish 130.
- Baseline A-B-C: last finish 150; idle 20. Both sequences meet all due times.
- With B release 120, A-C-B ends at 160; B tardiness is 10.
- With A duration 90 and original releases, A-C-B ends at 160; B is 10 minutes tardy.
Keep these three ideas
- Convert job data into a schedule with explicit time units and resource ownership.
- 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
Convert job data into a schedule with explicit time units and resource ownership.
- Introduce the specific Northstar inputs
- 1. For the given A-B-C sequence, A starts at 0 and ends at 60.
- A-B-C has 130 busy minutes, 20 idle minutes, and a last finish of 150.
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.