Overview
Post-mitigation ratings measure the residual risk after controls are in place. AIAG-VDA FMEA methodology requires both pre-mitigation (initial) and post-mitigation (final) risk assessments to verify that controls effectively reduce risk to acceptable levels. Severity typically remains unchanged (the effect is inherent to the failure), while Occurrence and Detection should improve through prevention and detection controls.
Prerequisites
- Failure modes analyzed with initial S-O-D ratings
- Risk controls linked to each failure mode or cause
- Controls implemented or planned with known effectiveness
Steps
1. Navigate to Post-Mitigation View
Open your FMEA document and select the Final Evaluation or Verify Controls view from the Risksheet view selector dropdown. This view displays the post-mitigation rating columns alongside initial ratings for comparison.
2. Re-Assess Occurrence
For each failure mode or cause with implemented controls, evaluate the new occurrence likelihood considering prevention controls:
- Locate the Occ New or Post-Mitigation Occurrence column
- Click the cell to open the rating dropdown
- Select the revised occurrence rating (0-10) based on control effectiveness
- Occurrence should decrease if prevention controls reduce failure likelihood
- Design change eliminating root cause: reduce from 8 to 2
- Statistical Process Control (SPC) on critical parameter: reduce from 6 to 4
- Poka-yoke device preventing assembly error: reduce from 7 to 2
- Redundant component: reduce from 5 to 3
3. Re-Assess Detection
Evaluate the new detection effectiveness considering added detection controls:
- Locate the Det New or Post-Mitigation Detection column
- Select the revised detection rating (0-10) based on improved controls
- Detection should decrease (better detection) if new controls improve failure visibility
Lower detection ratings = better detectability. Detection 1 (almost certain) is best, Detection 10 (almost impossible) is worst. If you added automated in-process detection, you might improve from Detection 8 (visual inspection) to Detection 3 (automated in-process).
| Initial Control | Initial Det | New Control | New Det |
|---|
| None (no testing) | 10 | Post-process gauging (100%) | 5 |
| Visual inspection (single) | 8 | Visual inspection (double-check) | 7 |
| Post-process SPC | 6 | In-process gauging (automated) | 3 |
| Post-process gauging | 4 | In-process with auto-stop | 2 |
4. Verify Severity Remains Constant
Post-mitigation Severity typically does not change because the effect of the failure mode is inherent to the design. Controls reduce the likelihood (Occurrence) or improve detection, but the consequence if it occurs remains the same.
Severity may decrease only if design controls fundamentally change the failure effect (e.g., adding a containment barrier that limits harm scope, downgrading from safety-critical to comfort function). In most cases, severity remains constant between initial and final ratings.
5. Review Auto-Calculated Post-Mitigation AP
The Action Priority New (AP New) or Post-Mitigation AP column automatically recalculates based on the revised S-O-D ratings. Traffic light indicators show:
- Green (Low): Risk mitigated to acceptable level
- Orange (Medium): Risk reduced but still requires monitoring
- Red (High): Controls insufficient, additional mitigation required
| Failure Mode | Initial AP | Final AP | Risk Reduction Status |
|---|
| FM-001 | H (180) | M (72) | Acceptable |
| FM-002 | H (240) | L (36) | Excellent |
| FM-003 | M (100) | H (160) | Worsened — review needed |
6. Identify Unresolved High-Risk Items
Use the traffic light panel in the top-right corner of the risksheet to check overall status:
- Pre light: Green when all initial assessments complete
- Post light: Green when all post-mitigation ratings entered
- High light: Red if any high-priority risks remain after mitigation
If the High light is red, filter the risksheet to show only items where AP New = H and define additional controls.
If post-mitigation AP is higher than initial AP, check for data entry errors. Occasionally this indicates that detection controls made the failure mode more visible (higher detection rating), revealing a more serious problem that requires design change.
Verification
You should now see:
- All post-mitigation Occurrence and Detection fields filled
- Auto-calculated post-mitigation Action Priority values
- Traffic light Post indicator showing green (all items assessed)
- Traffic light High indicator showing green (no unresolved high risks) or count of remaining high-priority items
- Row headers color-coded by final AP (green/orange/red) for visual risk status
Best Practices
- Complete initial ratings first: Post-mitigation ratings only make sense in comparison to baseline risk levels
- Document control rationale: Use the Prevention/Detection Control columns to justify why Occurrence/Detection improved
- Target high risks first: Prioritize re-assessment of initial High AP items to demonstrate risk reduction on critical failures
- Validate with test evidence: For detection improvements, link to verification test cases proving control effectiveness
- Review for completeness: Use the traffic light panel to ensure no items skipped in final evaluation
Common Issues
Post-mitigation AP unchanged despite controls: Verify that Occurrence and/or Detection ratings actually improved (decreased numerically). If controls are planned but not implemented, ratings should remain at initial values until verified effective.
Severity decreased inappropriately: Confirm that a design change fundamentally altered the failure effect, not just reduced likelihood. Severity changes should be rare and reviewed by the FMEA team.
Detection got worse (higher rating): Check if you accidentally confused the inverted scale. Detection 1 is best, 10 is worst. If detection truly degraded, investigate whether new controls introduced blind spots.
See Also
