Start with something familiar
Ordering a cake is not the same as having it ready at a party. The baker needs time to prepare it, a driver needs time to bring it, and you need time to set it out. A closed shop can add a day without anyone working more slowly. Work backward from the moment guests arrive. A supply planner uses the same approach, replacing hope with a dated chain of tasks and checking each handoff against its own working calendar.
Words to know: Lead time, Usable receipt.
Define the finish event
Northstar needs a part usable at the start of working day 10. Sequential lead-time stages are two working days at the supplier, one in transit, and one in receipt inspection. The total is four working days. Under the stated start-of-day convention, release at the start of day 6 gives usable receipt at the start of day 10.
The convention is part of the model. If the supplier starts work only the day after release, one more day is needed. If transit runs seven days but the warehouse closes weekends, stages need different calendars. Write the calendar beside each lead-time input. A date subtraction without a calendar can be arithmetically correct and operationally wrong.
Separate averages and commitments
A historical average of four days helps set a planning parameter, but it is not a commitment for a specific order. Current queues, holidays, transport bookings, and inspection capacity can change the date. Track quoted standard time, confirmed order time, and actual elapsed time as separate fields. This allows a buyer to explain whether the parameter or the individual execution failed.
A past-due recommended release date signals a decision. It does not authorize time travel. Choices include confirmed expediting, qualified alternate stock, an agreed later customer date, or a changed allocation. Each choice has a cost and an owner. Show both the original required date and the revised feasible date so the delay does not disappear when someone edits the promise.
Make lateness actionable
Calculate slack as required usable date minus feasible usable date in the chosen calendar. Positive slack is room; negative slack is lateness. Keep quantities attached to dates because a partial receipt may protect one order while leaving another exposed. State whether partial releases are operationally allowed.
In a dated planning table, distinguish scheduled receipts from new planned releases. A release creates an expected future receipt; it does not add stock immediately. Check that every receipt has an earlier authorized release or an existing order reference. This control catches plans that seem feasible only because new supply appears without elapsed time.
- Define units and event
- Reconcile eligible inputs
- Apply the stated rule
- Check feasibility
- Explain the decision
Northstar worked decision
All quantities and costs below are fictional and self-contained. Apply the stated timing and unit conventions.
| Input | Value |
|---|---|
| Required usable day | 10 |
| Supplier working days | 2 |
| Transit working days | 1 |
| Inspection working days | 1 |
- 1. Sum sequential times: 2 + 1 + 1 = 4 working days.
- 2. Release at start day 10 - 4 = 6.
- 3. If released start day 8, usable start day 12, two days late.
Base release day is 6.
Assumptions
- Use only the quantities and timing stated; no hidden receipts, losses, or substitutions.
- Fractional mathematical results are rounded only where whole-unit or pack rules are explicitly stated.
Where this can fail: This is a transparent teaching model. Confirm calendars, data definitions, and operating constraints before using an analogous policy in practice.
Build, test and explain a plan
Use numbered working days 1–20, all open, and the start-of-day convention. Bottle-part demand is 15 on day 10, 20 on day 14, and 10 on day 18; stock and backlog zero. Lead time is four working days, with lot-for-lot orders and unlimited capacity. Build all releases and receipts. The eight-hour allocation is a self-paced estimate, not experimentally validated; it includes model building, scenario testing, a memo, and teach-back.
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 reading and make a one-page definition sheet. Mark every unit, date convention, and inclusion rule. Explain the difference between the policy input and the outcome being measured. |
| 0.5 | Recalculate the worked example independently, without copying the displayed result. Show intermediate arithmetic and reconcile the answer to the physical story. Note one assumption that could change the decision. |
| 3 | Build a reusable multi-period spreadsheet from the supplied lab data. Use numbered working days 1–20, all open, and the start-of-day convention. Bottle-part demand is 15 on day 10, 20 on day 14, and 10 on day 18; stock and backlog zero. Lead time is four working days, with lot-for-lot orders and unlimited capacity. Build all releases and receipts. Keep inputs separate from formulas, add a clear output table, and include a check column that flags any quantity or timing inconsistency. Walk through the first and last rows manually. |
| 2 | Create separate named scenario tabs. Extend inspection by one working day for all orders; then impose a release ban before day 8 and report the first demand at risk. No expedited option exists in these scenarios. Preserve the baseline and record both changed inputs and changed outputs. Explain any outcome that stays the same, since a binding constraint or a measurement rule can make an intervention ineffective. |
| 1 | Write a 400-word decision memo naming your recommended action, evidence, assumptions, responsible role, and next check. Compare at least two alternatives using your table. State one limitation that could reverse the recommendation and what additional evidence would resolve it. |
| 1 | Teach the result aloud in five minutes using an original diagram. Then review every formula, answer the module questions relevant to this lesson, and write a short correction log. Use the remaining time to reproduce the key calculation from a blank sheet. |
Your deliverables
- An auditable multi-period workbook with baseline and named stress cases.
- A decision memo with numerical evidence and explicit limitations.
- An original explanatory diagram and a correction log.
Check your practice work
- Base release day is 6.
- One extra inspection day moves required release to 5.
- A day-8 release with four-day lead time misses day 10 by two working days.
- Base releases are fifteen on day 6, twenty on day 10, ten on day 14, with receipts on days 10,14,18.
- With total lead five, required release days are 5,9,13. A release ban before day 8 affects the first order: with base lead four its earliest usable date is 12.
Keep these three ideas
- Separate procurement, transit, inspection, and working calendars.
- Keep the unit, timing, and denominator visible.
- Treat model outputs as conditional on their stated assumptions.
Common mistakes
- Changing an input definition without changing all affected formulas.
- Presenting an expected or proposed quantity as confirmed usable supply.
Optional video study
Separate procurement, transit, inspection, and working calendars.
- Define the finish event
- Separate averages and commitments
- Make lateness actionable
First half: foundation and first worked check.
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.