Occurrence measures how likely a failure is to happen under normal operating conditions. Higher occurrence ratings indicate failure modes that happen frequently or with high probability, requiring prevention controls to reduce the likelihood.
FMEA Occurrence Rating Scale
The FMEA Occurrence scale ranges from 0 (Unanalyzed) to 10 (Very High), where each rating corresponds to a statistical failure probability and process capability index (Cpk) benchmark.
| Rating | Level | Probability Range | Failure Ratio | Cpk | Prevention Control Effectiveness |
|---|
| 0 | Unanalyzed | — | — | — | Not yet assessed |
| 1 | Remote | ≤ 1 in 1,500,000 | ≤ 0.00007% | ≥ 1.67 | Proven highly effective prevention |
| 2 | Very Low | 1 in 300,000 to 1 in 1,500,000 | 0.00007%–0.0003% | 1.50–1.67 | Proven effective prevention |
| 3 | Low | 1 in 50,000 to 1 in 300,000 | 0.0003%–0.002% | 1.33–1.50 | Proven effective prevention with controls |
| 4 | Low-Moderate | 1 in 20,000 to 1 in 50,000 | 0.002%–0.005% | 1.17–1.33 | Proven prevention with regular monitoring |
| 5 | Moderate | 1 in 2,000 to 1 in 20,000 | 0.005%–0.05% | 1.00–1.17 | Moderate prevention capability |
| 6 | Moderate-High | 1 in 500 to 1 in 2,000 | 0.05%–0.2% | 0.83–1.00 | Some proven prevention controls |
| 7 | High | 1 in 100 to 1 in 500 | 0.2%–1% | 0.67–0.83 | Limited prevention capability |
| 8 | Very High | 1 in 50 to 1 in 100 | 1%–2% | 0.50–0.67 | Prevention near-perfect but not proven |
| 9 | Very High | 1 in 5 to 1 in 50 | 2%–20% | 0.33–0.50 | No proven prevention available |
| 10 | Very High | ≥ 1 in 2 | ≥ 50% | ≤ 0.33 | No proven prevention |
Occurrence Categories
Unanalyzed (0)
- Default state when a failure mode is first created but not yet assessed
- Indicates incomplete FMEA analysis
- Should be highlighted in dashboard views and filtered from RPN calculations
- Must be resolved before document approval (per traffic light workflow validation)
Remote to Very Low (1–3)
- Statistical basis: Failure rate ratios from 1 in 1.5M down to 1 in 50K
- Automotive examples:
- Rating 1: Sensor detection hardware failure (redundant design proven effective)
- Rating 2: Connector corrosion in sealed enclosure (conformal coating verified effective)
- Rating 3: Mechanical fatigue in high-cycle component (material selection proven robust)
- Prevention strategy: Design robustness; proven prevention controls reduce risk to acceptable levels
- Cpk correlation: ≥ 1.33 indicates manufacturing process is well-controlled
Low to Moderate (4–6)
- Statistical basis: Failure ratios from 0.002% to 0.2% (1 in 50K to 1 in 500)
- Automotive examples:
- Rating 4: Electronic noise susceptibility mitigated by filtering
- Rating 5: Software logic error prevented by code review and unit testing
- Rating 6: Thermal stress from ambient temperature swings (within design envelope)
- Prevention strategy: Process controls and verification gates; regular monitoring required
- Cpk correlation: 1.00–1.17 indicates acceptable but not excellent process capability
High to Very High (7–10)
- Statistical basis: Failure ratios from 0.2% to ≥50% (1 in 500 to ≥ 1 in 2)
- Automotive examples:
- Rating 7: Recurring maintenance item (battery replacement every 3 years)
- Rating 8: Software patch deployment issues (prevention approach validated but unproven at scale)
- Rating 9: Operator error in assembly (no foolproof prevention method available)
- Rating 10: Consumable wear (inevitable over product lifetime; occurrence > 50%)
- Prevention strategy: Combination of design mitigation, process controls, detection improvements, and user training
- Cpk correlation: ≤ 0.67 indicates process capability is marginal; requires aggressive prevention
Occurrence-Based Severity Classification
The FMEA Occurrence scale is independent of severity—a high-occurrence failure mode can be harmless (low severity) or catastrophic (high severity). Severity is determined by the effect, not the likelihood.
Do not conflate Occurrence (how often?) with Severity (how bad?). A failure mode with occurrence = 10 (very likely) but severity = 1 (cosmetic) has lower overall risk (lower RPN) than a failure mode with occurrence = 2 (remote) and severity = 9 (critical injury).
Occurrence vs. Severity Decision Matrix
SEVERITY (Effect Seriousness)
Low (1-3) Medium (4-6) High (7-10)
OCCURRENCE
(Failure Likelihood)
Remote (1-2) ✓ Green ✓ Green 🟡 Yellow
Low Risk Acceptable Monitor
Moderate (4-6) ✓ Green 🟡 Yellow 🔴 Red
Acceptable Action Needed Immediate Action
High (7-10) 🟡 Yellow 🔴 Red ⛔ Critical
Action Needed Immediate Unacceptable Risk
Cpk (Process Capability Index) Relationship
The Cpk (process capability index) benchmarks in the occurrence scale reflect manufacturing process control maturity:
- Cpk ≥ 1.67 (Occurrence 1): Process is exceptionally robust; failures are virtually impossible under normal conditions
- Cpk 1.33–1.67 (Occurrence 2–3): Process is well-controlled; design margin is substantial
- Cpk 1.00–1.33 (Occurrence 4–6): Process is acceptable but requires active monitoring to stay in control
- Cpk 0.67–1.00 (Occurrence 7–8): Process is marginal; significant risk of going out of control; prevention controls essential
- Cpk ≤ 0.67 (Occurrence 9–10): Process is inadequate; failure mode is inherent or nearly unavoidable; mitigation through design or detection required
Implementation note: When assessing occurrence for design FMEA, correlate the rating to expected manufacturing process capability. If process Cpk can be improved through design (e.g., tighter tolerances, more robust materials), the occurrence rating may be reduced after risk controls are implemented.
RPN Calculation and Occurrence
The Risk Priority Number (RPN) is calculated as:
RPN = Severity × Occurrence × Detection
- Pre-mitigation RPN uses
premitigationFMOccurrence to establish baseline risk
- Post-mitigation RPN uses
postmitigationFMOccurrence to demonstrate risk reduction
- Occurrence improvements are achieved through prevention controls (design changes, process improvements, quality gates)
RPN Threshold Example
| Scenario | Severity | Occurrence | Detection | RPN | Action |
|---|
| High-risk mode (pre-mitigation) | 9 | 8 | 6 | 432 | ⛔ Unacceptable—requires mitigation |
| After prevention control | 9 | 4 | 6 | 216 | 🟡 Acceptable if post-mitigation reviewed |
| After prevention + detection | 9 | 4 | 3 | 108 | ✓ Low residual risk |
- Reducing Severity (design robustness)
- Reducing Occurrence (prevention controls)
- Reducing Detection (monitoring/inspection improvements)
Occurrence in Risksheet Configuration
Column Display and Editing
The occurrence enumeration populates editable dropdown columns in Risksheet FMEA configurations. Users select the occurrence rating from a picker list organized by category:
Cell Styling and Visual Indicators
Occurrence values can trigger conditional cell formatting to highlight risk levels at a glance:
| Occurrence Range | Cell Color | Icon | Meaning |
|---|
| 0 | Gray | ❓ | Not yet assessed; incomplete |
| 1–3 | Green | ✅ | Remote risk; well-controlled |
| 4–6 | Yellow | | Moderate risk; monitoring required |
| 7–10 | Red | ❌ | High risk; prevention urgently needed |
Progressive Workflow Views
Risksheet supports progressive FMEA views that enforce occurrence assessment before advancing to detection:
- Severity + Occurrence view — Users first rate severity and occurrence
- Validation gate — Unanalyzed items (0) are highlighted; cannot proceed until resolved
- RPN calculation — Once severity and occurrence are rated, RPN formula calculates automatically
- Detection + Mitigation view — Users then rate detection and select prevention controls
- Post-mitigation assessment — Post-mitigation occurrence is entered after controls are implemented
Assessing Occurrence: Practical Guidance
Step 1: Identify the Failure Mode Mechanism
Describe how the failure will occur (e.g., “Connector corrosion prevents signal transmission” or “Software memory leak exhausts available heap”).
Step 2: Estimate Failure Rate
Gather evidence:
- Field data: Historical failure rates from similar products or systems (e.g., X failures per million operating hours)
- Manufacturing data: Defect rates from production (e.g., Y% scrap due to this failure)
- Standards data: Industry benchmarks (e.g., ISO 26262 Annex G – Cpk-to-probability mappings)
- Physics modeling: Reliability calculations (e.g., Weibull analysis for wear-out failures)
- FMEA databases: Similar failure modes from previous FMEA projects
Step 3: Map to Occurrence Rating
Use the probability ranges in the scale above to select the appropriate rating:
Step 4: Cross-Check with Process Capability
Verify the rating is consistent with manufacturing process Cpk:
Cpk = (USL - mean) / (3 × σ) or (mean - LSL) / (3 × σ)
Step 5: Document Assumptions
Record the basis for the occurrence rating (e.g., “Based on 5-year field data showing 2 failures per 100K units in operation, rated as Occurrence 5 (Moderate)”).
Pre-Mitigation vs. Post-Mitigation Assessment
Pre-Mitigation Occurrence
Captures the baseline likelihood of the failure mode before any prevention controls are implemented:
- Reflects the inherent occurrence risk if no mitigation action is taken
- Used to calculate pre-mitigation RPN to identify high-priority failure modes
- Typically rated as the “as-is” process or design capability
Post-Mitigation Occurrence
Captures the reduced likelihood after prevention controls are implemented:
- Demonstrates effectiveness of risk mitigation strategies
- Used to calculate post-mitigation RPN to verify residual risk is acceptable
- Must be verified through test, analysis, or field trial before marking as complete
RPN Improvement Example
In Failure Mode work items, occurrence is captured in these custom fields:
premitigationFMOccurrence — Pre-mitigation baseline occurrence rating (0–10)postmitigationFMOccurrence — Post-mitigation reduced occurrence rating (0–10)fmSeverity — Effect severity (1–10; unchanged by mitigation)premitigationDetection — Pre-mitigation detection capability (0–10)postmitigationDetection — Post-mitigation detection capability (0–10)
Process FMEA Occurrence
For Process FMEA (PFMEA) failure modes, use the separate PFMEA Occurrence enumeration instead (0–5 injury probability scale), which focuses on manufacturing process risks.
Implementation in Polarion
Custom Field Definition
<custom-field id="premitigationFMOccurrence" name="Pre-Mitigation Occurrence">
<type>enum</type>
<enum id="fmOccurrence">
<!-- Enumeration values 0-10 as listed above -->
</enum>
<default>0</default>
<required>true</required>
</custom-field>
<custom-field id="postmitigationFMOccurrence" name="Post-Mitigation Occurrence">
<type>enum</type>
<enum id="fmOccurrence">
<!-- Enumeration values 0-10 as listed above -->
</enum>
<required>true</required>
</custom-field>
Risksheet Column Configuration
columns:
- id: premitigationOccurrence
label: "Pre-Mitigation Occurrence"
field: premitigationFMOccurrence
type: enum
width: 150
sortOrder: descending # High risk first
cellFormatter: occurrenceRiskColor # Apply color coding
- id: postmitigationOccurrence
label: "Post-Mitigation Occurrence"
field: postmitigationFMOccurrence
type: enum
width: 150
dependsOn: premitigationOccurrence # Progressive workflow
#set($rpn = $failureMode.fmSeverity * $failureMode.premitigationFMOccurrence * $failureMode.premitigationDetection)
Pre-Mitigation RPN: $rpn
See Also
