Probability & Impact Scoring

Consistent risk scoring enables objective comparison and prioritisation of risks. This guide provides detailed scales and criteria for assessing both probability and impact.

The Risk Score Formula

Risk Score = Probability x Impact
Where both Probability and Impact are rated on a scale (typically 1-5)

Probability Scale

Probability (or likelihood) measures how likely a risk is to occur.

5-Point Probability Scale

Score Rating Probability Range Description Indicators
5 Very High >80% Almost certain Has happened on similar projects, known issue
4 High 60-80% Likely Strong possibility based on experience
3 Medium 40-60% Possible Could happen, 50/50 chance
2 Low 20-40% Unlikely Not expected but possible
1 Very Low <20% Rare Would be surprising if it occurred

Probability Assessment Questions

When assessing probability, consider:

  • Has this happened before on similar projects?
  • What external factors could trigger this risk?
  • How much control do we have over this risk?
  • What is the track record of the parties involved?
  • Are there any early warning signs present?

Impact Scales

Impact measures the effect on project objectives if the risk occurs. Different impact dimensions may be assessed separately.

Cost Impact Scale

Score Rating Budget Impact Example
5 Very High >20% Major budget overrun, project viability threatened
4 High 10-20% Significant additional funding required
3 Medium 5-10% Manageable with contingency
2 Low 1-5% Minor cost increase
1 Very Low <1% Negligible financial impact

Schedule Impact Scale

Score Rating Schedule Impact Example
5 Very High >20% slip Major deadline missed, contractual penalties
4 High 10-20% slip Significant delay, milestone missed
3 Medium 5-10% slip Noticeable delay, recovery possible
2 Low 1-5% slip Minor delay, absorbed within float
1 Very Low <1% slip Negligible schedule impact

Scope/Quality Impact Scale

Score Rating Impact Example
5 Very High Deliverable unusable Product fails to meet core requirements
4 High Major functionality affected Key features compromised
3 Medium Some functionality affected Secondary features impacted
2 Low Minor reduction Nice-to-have features descoped
1 Very Low Barely noticeable Cosmetic impact only

Reputation/Strategic Impact Scale

Score Rating Impact Example
5 Very High Severe damage National media coverage, regulatory action
4 High Significant damage Major stakeholder relationship damaged
3 Medium Moderate damage Internal escalation, some stakeholder concern
2 Low Minor damage Limited visibility, quickly resolved
1 Very Low Negligible No stakeholder awareness

Combining Impact Dimensions

When a risk could affect multiple dimensions, there are several approaches:

Option 1: Highest Score

Use the highest impact score across all dimensions.

Risk Cost Schedule Quality Overall
R001 2 4 3 4
R002 5 2 1 5

Option 2: Weighted Average

Apply weights based on project priorities.

Dimension Weight R001 Score Weighted
Cost 40% 2 0.8
Schedule 40% 4 1.6
Quality 20% 3 0.6
Total     3.0

Option 3: Primary Dimension

Designate one dimension as primary based on project type.

Risk Score Interpretation

Using a 5x5 matrix, risk scores range from 1 to 25:

flowchart LR subgraph Low["LOW (1-4)"] L1[Accept or Monitor] end subgraph Med["MEDIUM (5-14)"] M1[Plan Response] end subgraph High["HIGH (15-25)"] H1[Urgent Action] end classDef blue fill:#108BB9,stroke:none,color:#fff class L1,M1,H1 blue
Score Range Risk Level Management Approach
1-4 Low Accept, monitor, or transfer to issue log if minimal
5-9 Medium-Low Assign owner, develop basic response plan
10-14 Medium-High Active management, contingency planning
15-19 High Escalate, urgent response required
20-25 Critical Executive attention, may require project pause

Worked Example

Project: Website redesign Risk: Third-party payment provider goes offline during launch week

Step 1: Assess Probability

  • Provider has had outages before: +1
  • Launch week is high-traffic period: +1
  • No recent stability issues reported: -1
  • Assessment: Medium probability (3)

Step 2: Assess Impact

| Dimension | Score | Rationale | |———–|——-|———–| | Cost | 3 | Lost sales, support costs | | Schedule | 4 | Launch delay possible | | Quality | 2 | Workaround available | | Reputation | 4 | Customer complaints |

Highest Impact: 4 (Schedule and Reputation)

Step 3: Calculate Score

Risk Score = 3 x 4 = 12 (Medium-High)

Step 4: Determine Response

Score of 12 requires active management and contingency planning.

Common Pitfalls

Anchoring Bias

Don’t let the first assessment influence others. Assess each risk independently.

Optimism Bias

Teams often underestimate probability. Challenge “it won’t happen to us” thinking.

Impact Underestimation

Consider knock-on effects. A schedule delay may also impact cost and reputation.

Inconsistent Scales

Ensure everyone uses the same definitions. One person’s “medium” shouldn’t be another’s “high”.

Static Assessments

Probability and impact change over time. Reassess regularly.

Calibration Techniques

Reference Class Forecasting

Compare to similar past projects. If 3 of 10 similar projects had this issue, probability is ~30%.

Delphi Technique

Gather independent assessments from multiple experts, then discuss and converge.

Pre-Mortem Analysis

Imagine the risk has occurred. Work backwards to assess how likely that scenario is.

Quantitative vs Qualitative

Approach When to Use Pros Cons
Qualitative (1-5 scales) Most projects Quick, simple, intuitive Subjective, less precise
Quantitative (% and £) Large/complex projects Precise, enables EMV Time-consuming, requires data

Expected Monetary Value (EMV)

For quantitative analysis: EMV = Probability (%) x Impact (£)

Example: 30% chance of £50,000 cost overrun = EMV of £15,000

Last updated: 13 January 2026