risk-matrix

Overview

Per rbt-wiki:

"Risk-based testing (RBT) is 'a type of software testing that functions as an organizational principle used to prioritize the tests of features and functions in software, based on the risk of failure.'"

The risk matrix is the artifact that drives RBT decisions. Without it, prioritization is gut-feel; with it, the team has a defensible record of "why we tested this and not that."

When to use

• A new feature is in planning; the team needs to scope test effort.
• A release scope is set; the team needs to allocate manual + auto test time.
• Quarterly: the team wants to know which areas have the highest unmitigated risk.
• Audit / compliance: the team must show a documented risk assessment.

How to use

1. Pick the methodology (Step 1) - lightweight impact-by-likelihood for most teams; heavyweight FMEA / Cost of Exposure only when regulation demands it.
2. Intake each risk (Step 2) - one row per risk with feature, category, impact 1-5, likelihood 1-5, and the resulting score.
3. Plot the heatmap and tier the verdict (Step 2) - block / mitigate-this-sprint / accept, using the team's block threshold (typically score 15+).
4. Tag categories (Step 3) - business, technical, regulatory, UX, security, performance, integration - so patterns surface over time.
5. Read the test plan off the matrix (Step 4) - map each top risk to its recommended test types.
6. Set the cadence and store it in git (Steps 6-7) - the matrix is a living document reviewed per feature, per release, and quarterly.

The worked example below runs a checkout release through the full flow.

Step 1 - Pick the methodology

Per rbt-wiki, two approaches:

Most teams should start lightweight - heavyweight methods need specialized expertise.

Step 2 - Lightweight matrix structure

# Risk matrix - `<feature/release>`

**Date:** YYYY-MM-DD   **Owner:** _______________   **Reviewers:** _______________

## Risks

| ID  | Risk                                          | Category   | Impact (1-5) | Likelihood (1-5) | Score | Mitigation                          | Owner | Due |
|-----|-----------------------------------------------|------------|-------------:|-----------------:|------:|-------------------------------------|-------|-----|
| R-1 | Promo discount math wrong (off-by-cent)        | Business   |      5       |       3          |  15   | Add property-based tests on rounding | Alice | 2026-05-15 |
| R-2 | Stripe webhook delivery failure not retried    | Technical  |      4       |       4          |  16   | Add retry + DLQ; add chaos test     | Bob   | 2026-05-12 |
| R-3 | EU tax calculation incorrect                    | Regulatory |      5       |       2          |  10   | UAT with finance team               | Carol | 2026-05-20 |
| R-4 | Cart loses state on app restart                 | UX         |      3       |       3          |   9   | Persistent cart in localStorage     | Dave  | 2026-05-10 |

## Heatmap

| Likelihood ↓ \ Impact → | 1 (low) | 2 | 3 | 4 | 5 (high) |
|--------------------------|---------|---|---|---|----------|
| 5 (very likely)           |         |   |   |   |          |
| 4                          |         |   |   | R-2 |        |
| 3                          |         |   | R-4 |   | R-1     |
| 2                          |         |   |   |   | R-3     |
| 1                          |         |   |   |   |          |

## Verdict

- **Critical (score >=15):** R-1 (15), R-2 (16). Block release until mitigated.
- **High (score 9-14):** R-3 (10), R-4 (9). Mitigate this sprint.
- **Medium (score 5-8):** (none).
- **Low (score 1-4):** (none).

The 5×5 matrix yields scores 1-25; the team picks the threshold for "block release" (typically 15+).

Step 3 - Risk categories

Per rbt-wiki, risks span:

• Business/operational: System criticality and usage frequency
• Technical: Team distribution and complexity
• External: Regulatory requirements and stakeholder preferences
• E-business specific: Security vulnerabilities, performance failures, and integration defects

Tag every risk with one category. Patterns emerge over time - "all our top risks are integration" suggests an architectural review, not just more testing.

Step 4 - Map risks to test types

A populated matrix drives the test plan:

The test plan reads off the matrix: top-N risks → test types per risk → estimated effort.

Worked example - checkout redesign release

Input: the checkout-redesign release scope, lightweight methodology (Step 1).

Intake and score (Steps 2-3). Four risks surface, each tagged with a category and scored impact × likelihood:

Tier the verdict (Step 2). With a block threshold of 15: R-2 (16) and R-1 (15) block the release until mitigated; R-3 (10) and R-4 (9) are high and get mitigated this sprint.

Read the test plan off the matrix (Step 4). Each risk's category selects its test types:

• R-1 (business logic) -> unit + property-based tests on rounding, plus UAT on the promo path.
• R-2 (technical / integration) -> retry + DLQ, a chaos test on webhook delivery, and contract tests against Stripe.
• R-3 (regulatory) -> UAT with the finance team plus a compliance review of the EU tax rules.
• R-4 (UX) -> manual exploratory plus visual-regression coverage of the persisted cart.

Outcome. The two blocking risks (R-1, R-2) get owners and due dates before sign-off; the matrix is committed to docs/risk-matrices/2026-Q2-checkout-redesign.md and re-reviewed at the next cadence point (Step 6).

Step 5 - Heavyweight methods (FMEA, Cost of Exposure)

For regulated or safety-critical products where lightweight scoring is insufficient, risk-based testing offers quantitative methods - FMEA (Risk Priority Number = severity × occurrence × detectability) and Cost of Exposure (annual financial risk vs mitigation cost) - per rbt-wiki. Both need specialized expertise; use them only when regulation or financial justification requires it. Full row structure, worked FMEA and Cost-of-Exposure tables, and a when-to-use guide are in references/heavyweight-risk-scoring.md.

Step 6 - Cadence

The matrix is a living document - risks change as features ship, mitigations land, and incidents reveal new failure modes.

Step 7 - Format + storage

docs/risk-matrices/
├── 2026-Q2-checkout-redesign.md
├── 2026-Q2-stripe-integration.md
├── 2026-Q1-summary.md       ← rollup
└── README.md

Markdown files version-controlled in git. Reviews via PR; updates tracked over time.

Anti-patterns

Limitations

• Subjective scoring. Even with a rubric, team members weigh risks differently. Multiple-reviewer consensus helps.
• Doesn't predict unknown unknowns. RBT addresses identified risks; new failure modes emerge from incidents (and update the matrix).
• Heavyweight methods need expertise. FMEA is well-defined; QFD / FTA require specialized training.

References

• rbt - Risk-based testing definition, lightweight vs heavyweight methods (FMEA, Cost of Exposure, QFD, FTA), risk categories (business / technical / external / e-business).
• risk-storming-facilitator - sibling: facilitates the session that fills the matrix.
• test-strategy-author - test strategy doc that references the matrix.