Prerequisites
- Mitigation actions assigned and implemented (see Add DFMEA Risk Mitigation Actions)
- Pre-mitigation RPN scored for comparison (see Score Severity, Occurrence, and Detection (RPN))
Understanding Post-Mitigation RPN
Post-mitigation RPN measures the residual design risk after corrective and preventive actions are in place. The formula is identical to pre-mitigation: Post-RPN = S x O(new) x D(new)
Step 1: Assign Post-Mitigation Occurrence (O-new)
For each failure mode cause with an assigned mitigation:- Click the Post-Mitigation Occurrence column (
postmitigationFMOccurrence) - Select the updated occurrence rating reflecting conditions after mitigation:
| O Value | Label | Probability Range | Question to Ask |
|---|---|---|---|
| 0 | Unanalyzed | — | Has the mitigation been evaluated? |
| 1 | Never | < 5% | Has the design change eliminated the failure mechanism? |
| 2 | Seldom | 5-25% | Has the mitigation significantly reduced likelihood? |
| 3 | Sometimes | 25-75% | Has the mitigation moderately reduced likelihood? |
| 4 | Often | 75-95% | Has the mitigation only marginally reduced likelihood? |
| 5 | Always | > 95% | Has the mitigation had no effect on likelihood? |
Step 2: Assign Post-Mitigation Detection (D-new)
For each failure mode cause:- Click the Post-Mitigation Detection column (
postmitigationDetection) - Select the updated detection rating:
| D Value | Label | Probability Range | Question to Ask |
|---|---|---|---|
| 0 | Unanalyzed | — | Has detection capability been evaluated? |
| 1 | Never | < 5% | Can current verification detect this failure? |
| 2 | Seldom | 5-25% | Are new inspection steps or tests in place? |
| 3 | Sometimes | 25-75% | Are monitoring systems partially effective? |
| 4 | Often | 75-95% | Are automated detection mechanisms in place? |
| 5 | Always | > 95% | Is the failure reliably caught before reaching the user? |
Adding verification test cases, automated inspections, or built-in self-test (BIST) capabilities typically improves the detection rating. Link these verification measures through the requirements traceability columns.
Step 3: Review the Post-Mitigation RPN
The risksheet automatically computes the post-mitigation RPN using thecommonRpnNew formula:
Post-RPN = S x O(new) x D(new)
The rpnNew column updates with the new value, and the row header color updates to reflect the post-mitigation risk level.
Post-RPN Color Coding
| RPN Range | Color | Risk Level | Interpretation |
|---|---|---|---|
| 1-10 | Green (rpn1) | Low | Mitigation is effective; risk is acceptably low |
| 11-30 | Amber (rpn2) | Medium | Mitigation provided some reduction; consider further action |
| > 30 | Red (rpn3) | High | Mitigation is insufficient; additional actions needed |
Step 4: Compare Pre and Post RPN
Review the effectiveness of your mitigations by comparing pre and post values:| Comparison | Interpretation | Action |
|---|---|---|
| Post-RPN < Pre-RPN | Mitigation reduced risk | Document and accept |
| Post-RPN = Pre-RPN | Mitigation had no measurable effect | Reassess the mitigation strategy |
| Post-RPN > Pre-RPN | Scoring error or unexpected change | Review O and D ratings |
| Failure Mode | S | O (pre) | D (pre) | Pre-RPN | O (post) | D (post) | Post-RPN | Reduction |
|---|---|---|---|---|---|---|---|---|
| Incorrect dosage | 5 | 3 | 2 | 30 | 1 | 4 | 20 | 33% |
| Occlusion missed | 4 | 4 | 1 | 16 | 4 | 4 | 64 | — (detection improved) |
| Inaccurate display | 3 | 2 | 3 | 18 | 1 | 5 | 15 | 17% |
Since higher Detection values indicate better detection capability but also increase the RPN product, evaluate the individual factors alongside the total RPN. A high D-new (good detection) with low O-new (rare occurrence) is a positive outcome even if the raw RPN is moderate.
Step 5: Iterate on High Residual RPN
For failure modes where the post-mitigation RPN remains in the red zone (>30):- Review whether additional mitigation actions are feasible
- Consider whether a design change (reducing occurrence) would be more effective than improving detection
- Add new mitigation tasks — see Add DFMEA Risk Mitigation Actions
- Re-score after implementing additional measures
Step 6: Document Final Assessment
After all post-mitigation scores are complete:- Verify every failure mode has non-zero post-mitigation O and D values
- Confirm the row header colors reflect the intended risk levels
- Check HARA cross-references to ensure consistent risk assessment across both methodologies
- Generate the DFMEA Summary Report for stakeholder review
Related Resources
- Score Severity, Occurrence, and Detection (RPN) — pre-mitigation scoring reference
- Add DFMEA Risk Mitigation Actions — creating and linking tasks
- DFMEA Risksheet Configuration Reference — column and formula details
- Read the DFMEA Summary Report — reviewing the aggregate analysis
Sources
Sources
DFMEA risksheet configuration (
DFMEATemplate/risksheet.json), failure mode custom fields (failureMode-custom-fields.xml), occurrence enumeration (fmOccurrence-enum.xml), detection enumeration (detection-enum.xml), DFMEA top panel formulas (DFMEATemplate/risksheetTopPanel.vm), UI walkthrough (risksheet-views.md).