PRINCE2's Risk Management Approach requires the Project Manager to define probability and impact scales as part of the Project Initiation Document. These scales are then used consistently throughout the project to assess every risk in the Risk Register. While PRINCE2 requires these scales to be defined, it provides limited guidance on what the impact definitions should actually say — this post fills that gap.
Probability Scale
Probability measures how likely the risk is to occur. A 5-level scale expressed as percentage ranges:
| Level | Probability Range |
|---|---|
| Very Likely | 80–99% |
| Likely | 60–79% |
| Possible | 40–59% |
| Unlikely | 20–39% |
| Very Unlikely | 1–19% |
Impact Scale
Impact measures the effect the risk would have on the project's objectives if it occurred. Defining impact in terms of project objectives (rather than generic descriptions) makes the assessment more meaningful:
| Level | Description |
|---|---|
| Severe | One or more critical project objectives will not be achieved |
| Significant | Project objectives will fall below acceptable thresholds |
| Moderate | Objectives fall below goals but remain above minimum acceptable levels |
| Minor | Objectives fall short of targets but remain well above minimums |
| Negligible | Little to no effect on achieving project objectives |
The Risk Matrix
Combining probability and impact ratings in a 5×5 matrix creates a visual risk map. Risks in the top-right corner (high probability, high impact) require immediate response plans. Risks in the bottom-left (low probability, low impact) can be monitored and reviewed at regular intervals. The Risk Management Approach should define threshold levels at which risks are escalated to the Project Board.
See the PRINCE2 Risk Practice for the full five-step risk management procedure.