Why is the textbook schedule too optimistic?
Two parallel branches merge at integration, and integration waits for the slower of two uncertain paths. Averages ignore that; twenty thousand simulated schedules do not.
Words on this sheet
- Predecessors: The tasks that have to finish before this one can start. Put their IDs here, separated by commas, and leave it blank when nothing has to come first.
- F statistic: The variation between the groups divided by the variation inside them.
School Starter Schedule Risk Pro engine
After you install, this is the model to open.
The Classic Project Network, Simulated
- In your spreadsheet, click the Sortia icon in the strip of icons down the right-hand edge. No strip? Click the arrow at the bottom-right to open it. You can also use Extensions, then Sortia, then Open Sortia.
- Click Start from a template and put that name in the search box.
- Pick the card with that name and click Load this template. It arrives on a new tab with real numbers already in it.
This one runs on a Pro engine, and every free install includes five full-quality runs on your own numbers, shared across all five Pro engines rather than five for each. After that, Pro is $199/year.
The answer
Twenty thousand simulated schedules:
- Chalkboard answer
- 21 weeks
- P50 / P80
- 21.4 / 23.0 weeks
- 22-week promise holds
- 62% of trials
The PERT point estimate is not wrong, it is incomplete: a 22-week promise holds in only 62 trials of 100 because of merge bias, and the criticality table shows the software branch driving nearly every trial. Compare the chalkboard answer with the simulated one on any problem set.
The model
The course-staple network: requirements, then design-and-build in parallel with order-and-install, merging at integrate-and-test, ending in training. Every duration a three-point estimate in weeks.
| Activities | 7, two parallel branches |
| Durations | optimistic / likely / pessimistic |
| Finish distribution | simulated |
| Criticality | simulated |
Once it is in your sheet
- The model arrives with real numbers in it and runs as it stands, so you can press the button first and understand it second.
- Change the numbers to yours. The sheet marks which cells are inputs and which hold formulas, and most labels carry a note explaining the row.
- Press the run button at the bottom of the panel. It is labeled for the tool you are in, and the result lands on its own tab, with a written reading of it beside the figures.
Never used Google Sheets? Start here goes the whole way, in seven steps, and assumes nothing.
Next question
- Does discounting to fill the room ever work?What Does the Course Have to Charge?
- Is an equal share of the work actually fair?Share the Marking Fairly
- Did the course work or do the students just differ?Did the Course Move the Scores?
- Would more places actually fix the shortfall?Will the Programme Cover Its Costs?
- Is the cheapest fix on your risk list the best one?What Could Go Wrong This Term?
- Which class should go to the hired hall?Fit the Timetable Without a Clash
Every model like this one, and the method behind them: Schedule risk analysis.