Start with something familiar
A bus service can report the last bus home, the average wait, or the largest delay. Each is true, but each answers a different question. A route that gets one bus home early can still leave one passenger waiting a long time. Likewise, a factory can finish the whole day at the same time under two plans while treating customers quite differently. A useful scorecard names the experience it measures instead of calling every result speed.
Words to know: Lateness, Tardiness, Makespan, Release time.
Choose the right subtraction
Completion is measured from the shared time origin. Flow time is completion minus release time, so it measures time spent in the system after the job becomes ready. If jobs have different releases, average completion and average flow do not have identical numeric values. Preserve release data instead of assuming every job has waited since the shift began.
Lateness is signed. A job finishing ten minutes before its promise has lateness minus ten. Tardiness is never negative: it counts only delay after the promise. When you average signed lateness, early jobs can hide a late job. That might be useful for a particular analysis, but it should never be labeled average tardiness.
The last finish does not explain service
Makespan in this course uses the declared zero clock and the latest finish. If total run time is fixed and there are no idle gaps or setups, changing the sequence cannot change that latest finish. It can still change every individual completion. Do not promise a shorter shift merely because the average job finishes sooner.
A weighted tardiness score multiplies each job delay by a declared weight. Those weights encode priorities; they are not physical minutes of work. Report the units as weighted minute points and state the policy behind them. A larger weight can justify attention, but it never makes an infeasible start time legal.
Read the distribution
Include the count of tardy jobs and the worst delay alongside the total. A total of sixty could mean one order is sixty minutes late or six orders are ten late. Those situations can need different customer actions. Use a job-level table as the audit trail behind the summary so the affected order remains visible.
When scores disagree, write a decision sentence: choose this feasible plan because it meets the named priority, while accepting this stated loss. Avoid a vague claim that it is optimal. For the small lab you can enumerate every sequence, but even a complete enumeration proves best performance only for the model and objective you actually defined.
- 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 | Start | Duration | Release | Due | Weight |
|---|---|---|---|---|---|
| A | 0 | 50 | 0 | 40 | 3 |
| B | 50 | 30 | 10 | 100 | 1 |
| C | 80 | 20 | 0 | 90 | 2 |
- 1. Completions are A 50, B 80, C 100. Flow times are 50,70,100.
- 2. Lateness is 10,-20,10; tardiness is 10,0,10.
- 3. Weighted tardiness is 3×10 + 1×0 + 2×10 = 50 points.
Makespan 100; mean flow 73.33; total tardiness 20; two jobs tardy; maximum lateness 10.
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 processing times, releases, dues, and weights on one machine open 0-200, with no setups. Enumerate six earliest-start sequences. Show idle gaps forced by releases. Select minimum total tardiness and minimum weighted tardiness separately. Stress A weight 10, then stress B release 60 with original weights. Give all calculations so competing choices can be reviewed. 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
- A-C-B completes A50,C70,B100, giving tardiness 10,0,0 and weighted score 30.
- C-A-B completes C20,A70,B100, giving tardiness 0,30,0 and weighted score 90.
- B-C-A starts B10 and finishes B40,C60,A110; A tardiness 70 and weighted score 210.
- With B release60, A-C-B still finishes at100 and weighted score30. With A weight10 its score is100.
Keep these three ideas
- Separate lateness, tardiness, flow time, and makespan in schedule decisions.
- 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
Separate lateness, tardiness, flow time, and makespan in schedule decisions.
- Introduce the specific Northstar inputs
- 1. Completions are A 50, B 80, C 100. Flow times are 50,70,100.
- Makespan 100; mean flow 73.33; total tardiness 20; two jobs tardy; maximum lateness 10.
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.