ModelRisk example · General techniques
Simple Project Schedule with Risks
Why does a project built from most-likely durations finish late? Sixteen activities, three paths, three risks.
From the model’s saved run
When will the project finish?
The finish week against the plan (most likely durations, no risks) and the contract.
| Measure | Unit | Value |
|---|---|---|
| Planned finish (most likely durations, no risks) | weeks | 68.0 |
| Mean finish (simulated) | weeks | 78.9 |
| Chance of finishing by the planned week | probability | 0.1% |
| Contract completion | weeks | 80.0 |
| Chance of meeting the contract date | probability | 61.5% |
S-curve (cumulative probability) of Finish, Plan, Contract from the saved simulation.
The business question
A company is building a small factory. The schedule has sixteen activities. After design and approvals, three activities run side by side, and then three paths follow: the building itself (foundations, steel frame, roof), the site services (ground works, drainage, utility connections) and the process equipment (order, manufacture, installation). Commissioning and fit-out can start only when all three are done, followed by the handover tests.
Each duration is uncertain, between 80% and 150% of its most likely value. Ground works can go three ways: as normal, with a problem, or better than planned.
Three risks can add delay: storm damage to the cladding, a utility company that connects late, and an installation crew that loses its slot, which is twice as likely if one of the other two has already delayed the site. The plan, with every activity at its most likely duration and no risks, finishes in week 68, and the contract allows 80 weeks.
- Who it is for
- Project planners and managers who build schedules from single-point estimates, and anyone new to schedule risk analysis.
- Main outputs
- The finish week and date at P50 / P80 / P90 · the chance of meeting the plan and the contract date · how often each path is critical · how often each risk happens · the activities that drive the finish.
How the model works
Three sheets after this one, read from left to right. Each one feeds the next.
Inputs sheet
The date the project starts and the contract completion date.
Schedule sheet
Each activity starts when the last of its predecessors finishes (MAX) and lasts a triangular duration. Ground works' duration depends on which of its three ways happens (VoseDiscrete). Each risk happens with its probability (VoseBernoulli) and adds its delay to the activity it affects. The crew risk's probability is raised if either other risk happened.
The plan column repeats the logic with most likely durations and no risks.
Results sheet
The finish week and date at P50 / P80 / P90 against the plan and the contract, how often each path is the critical one, how often each risk happens, and the activities that drive the finish.
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The results in detail
Finish dates
P50 / P80 / P90: a 50%, 80% and 90% chance of finishing by then.
| Measure | Unit | P50 | P80 | P90 |
|---|---|---|---|---|
| Finish | weeks | 78.8 | 82.3 | 84.2 |
| Finish date | date | 8 Jul 2028 | 2 Aug 2028 | 15 Aug 2028 |
Which path is critical
How often each of the three paths finishes last and so sets the start of commissioning.
| Path | Probability |
|---|---|
| Roof path (E → F → G) | 58.1% |
| Utilities path (H → I → J) | 8.6% |
| Equipment path (K → L → M) | 33.3% |
| Total | 100.0% |
The risks
How often each risk happens in the simulation, and the delay it adds on average over all iterations.
| Risk | Probability | Happens | Mean delay |
|---|---|---|---|
| R1 Storm damage to the cladding (on G) | 30% | 30.8% | 1.23 |
| R2 Utility company connects late (on J) | 50% | 50.5% | 2.19 |
| R3 Installation crew loses its slot (on M) | 20% | 33.4% | 1.89 |
Which activities matter
Lowest and highest mean finish over ten equal groups of each activity's duration (VoseSimSensitivityMean).
Tornado chart: the inputs ranked by how far the mean output moves across each input’s range.
The ranking as a table 10 inputs
| Input (ranked by influence) | Lowest mean | Highest mean | Rank |
|---|---|---|---|
| A Design and approvals | 76.8 | 81.5 | 1 |
| C Procure structural steel | 77.5 | 81.7 | 2 |
| G Roof and cladding | 77.4 | 81.2 | 3 |
| P Handover tests | 77.3 | 81.0 | 4 |
| N Commissioning | 78.0 | 81.0 | 5 |
| O Fit-out | 77.9 | 80.8 | 6 |
| F Steel frame | 78.0 | 80.5 | 7 |
| B Site clearance | 78.1 | 80.5 | 8 |
| E Foundations | 78.2 | 80.3 | 9 |
| M Equipment installation | 78.3 | 80.2 | 10 |
How to read these results
- The plan, with every activity at its most likely duration and no risks, finishes in week 68, and the simulation almost never achieves it. Three things push the finish out: each duration can run to 150% of its most likely value but only down to 80%. Commissioning waits for the slowest of three paths; and the risks add delay.
- Plan on the P80 date rather than the most likely one. The contract's 80 weeks give about a 60% chance of finishing on time.
- The roof path is critical in about six iterations out of ten and the equipment path in about three. A plan that watches only the roof path misses a third of the ways the project can be late.
- The installation crew's risk is entered at 20% but happens about one time in three: it rises to 40% whenever storm damage or the utility company has already delayed the site, and the utility risk happens half the time.
See it move. Open the workbook, change an input on the Inputs sheet and run the simulation: every table and chart on this page updates in Excel.
Inputs and assumptions
Every assumption in the model, with its uncertainty and where it comes from. The figures are illustrative.
Dates 2 inputs
When the project starts, and the completion date in the contract.
| Assumption | Unit | Value |
|---|---|---|
| Project start dateA Monday. | date | 4 Jan 2027 |
| Contract completion date80 weeks after the start: 12 weeks more than the plan. | date | 17 Jul 2028 |
Units and conventions
- Time
- Weeks from the project start; dates are the start date plus 7 days per week.
- Plan
- Every activity at its most likely duration, ground works as normal, no risks.
- Critical path
- The path among the three that finishes last in an iteration: it sets the start of commissioning.
Key assumptions and limitations
- Finish-to-start
- An activity starts as soon as all its predecessors have finished; no lags, no resource limits.
- Durations
- Triangular from 80% to 150% of the most likely value, as in the original example.
- Risk delays
- Added to the duration of the activity a risk affects. Storm damage adds 20% to 50% of the roof's duration.
- Data
- The project is invented for this example. The network, durations and risks are those of ModelRisk's earlier Schedule Model With Risks example, and ground works' three ways those of Project Schedule 3.
How to use it
- Set the start and contract dates on the Inputs sheet.
- Replace the activities, their durations and 'starts after' links on the Schedule sheet: a new link is a new reference in the MAX of the Start column.
- Run a simulation from the ModelRisk ribbon (10,000 samples with a fixed seed are set).
- Use the P80 date rather than the plan for commitments, and the critical-path shares to decide where to act.
The tornado chart fills in after a simulation with ModelRisk’s classic engine.
ModelRisk functions used
- VoseTriangle
- Each activity's duration, and each risk's delay if it happens.
- VoseDiscrete
- Which way ground works go: normal, with a problem, or better than planned.
- VoseBernoulli
- Whether each risk happens; the crew risk's probability depends on the other two.
- VoseInput, VoseOutput
- Name each activity's duration and the finish week, for the Results Viewer and the tornado.
- VoseSimMean, VoseSimPercentile
- Statistics of the simulation: chances (means of 1/0 outcomes) and finish dates.
- VoseSimSensitivityMean
- The tornado chart: the activities whose durations move the mean finish most.
Further reading
- Sensitivity analysis in Excel: a tornado chart step by step (Vose guide)
- Vose, D. (2008). Risk Analysis: A Quantitative Guide, 3rd edition. Wiley: project risk analysis.
- Hulett, D. (2009). Practical Schedule Risk Analysis. Gower: why parallel paths make projects late.
Download the workbook and run it yourself. It opens in Excel with ModelRisk. The free trial runs every example model.
Related example models
- ModelRisk★★★★★Correlation Through a Common Factor (Look-up Table)How much later does a project finish when bad weather hits every activity at once?One shared random factor and a look-up table make six durations move together, without a correlation matrix.
- ModelRisk★★★★★A Step Change in the Event Rate (Projected Cures)How many lives will a cure save if nobody knows when it will arrive?A Poisson series with a trend, thinned by a binomial from an uncertain month on.
- ModelRisk★★★★★Bounded Random WalkWhat does a growing, volatile series look like when practical limits hold it in?A lognormal random walk held between a rising cap and a falling floor, each period starting from the bounded value.
An illustrative example: the figures are invented for teaching and do not describe a real company, project or asset. Last updated 9 October 2026.