Overview
The PFMEA Occurrence scale ranges from 0 to 10, where each rating represents a progressively higher probability of cause occurrence. Unlike severity (which remains constant regardless of controls), occurrence ratings change after process control implementation, enabling measurement of risk reduction effectiveness.
| Rating | Definition | Probability Range | Typical Cpk Range | Example Scenario |
|---|
| 0 | Unanalyzed | Unknown | Not applicable | New cause requiring assessment |
| 1 | Remote | 1 in 1,500,000 | ≥ 1.67 | Error-proofed process, systematic prevention |
| 2 | Low | 1 in 500,000 | 1.33-1.67 | Robust design with mistake-proofing |
| 3 | Low | 1 in 200,000 | 1.33-1.67 | Well-controlled process with SPC |
| 4 | Moderate | 1 in 50,000 | 1.0-1.33 | Controlled process with periodic inspections |
| 5 | Moderate | 1 in 20,000 | 1.0-1.33 | Basic statistical control |
| 6 | Moderately High | 1 in 10,000 | 0.67-1.0 | Manual process controls in place |
| 7 | High | 1 in 5,000 | 0.67-1.0 | Inadequate controls, frequent occurrence |
| 8 | High | 1 in 2,000 | < 0.67 | Minimal controls, predictable failure |
| 9 | Very High | 1 in 500 | < 0.67 | No preventive controls, regular occurrence |
| 10 | Almost Certain | > 1 in 500 | < 0.67 | Virtually certain without intervention |
Enumeration Properties
| Property | Value | Description |
|---|
| Enumeration ID | pfmOccurrence | Internal identifier used in risksheet and powersheet configurations |
| Field Type | Integer Enum | Numeric scale constrained to 0-10 range |
| Default Value | 0 | Indicates unassessed cause; must be set before RPN calculation |
| Mutability | Pre/Post | Separate fields for initial (premitigationPFMOccurrence) and post-control (postmitigationPFMOccurrence) ratings |
| Related Work Items | Process Failure Mode, Process Step | Used to assess root causes in PFMEA documents |
| Integration | RPN Formula | Part of RPN calculation: RPN = Severity × Occurrence × Detection |
Technical Implementation
Custom Field Bindings
PFMEA Occurrence is captured through dedicated custom fields in Process Failure Mode work items:
<!-- Pre-Mitigation Occurrence (Initial Risk Assessment) -->
<field name="premitigationPFMOccurrence"
type="enum"
enumType="pfmOccurrence"
label="Pre-Mitigation Occurrence"
required="true" />
<!-- Post-Mitigation Occurrence (After Control Implementation) -->
<field name="postmitigationPFMOccurrence"
type="enum"
enumType="pfmOccurrence"
label="Post-Mitigation Occurrence"
required="false" />
Risksheet Configuration
The PFMEA Risksheet configuration binds occurrence ratings to Level 3 (Cause) rows, enabling per-cause occurrence assessment:
columns:
- id: "pfmOccurrencePre"
type: "enum"
enumeration: "pfmOccurrence"
label: "Occurrence"
bind: "premitigationPFMOccurrence"
level: 3
columnGroup: "Set Ranking"
width: 80
- id: "pfmOccurrencePost"
type: "enum"
enumeration: "pfmOccurrence"
label: "Post-Occ"
bind: "postmitigationPFMOccurrence"
level: 3
columnGroup: "Final Ranking"
width: 80
Rating Criteria and Assessment
Occurrence ratings must align with manufacturing capability data. Reference Statistical Process Control (SPC) charts, defect histories, and process capability (Cpk) indices to justify ratings rather than guessing.
Rating Selection Decision Matrix
START: Assess the root cause frequency
┌─────────────────────────────────────────┐
│ Has error-proofing prevented the cause? │
│ (Poka-yoke / Mistake-proofing) │
└─────────────────────────────────────────┘
YES → Rating: 1-2 (Remote)
NO → Continue
┌─────────────────────────────────────────┐
│ Is Statistical Process Control (SPC) │
│ actively monitoring this process? │
│ (Cpk ≥ 1.33?) │
└─────────────────────────────────────────┘
YES → Rating: 3-4 (Low to Moderate)
NO → Continue
┌─────────────────────────────────────────┐
│ Are manual inspections or audits │
│ performed regularly? │
└─────────────────────────────────────────┘
YES → Rating: 5-6 (Moderate to M-High)
NO → Continue
┌─────────────────────────────────────────┐
│ Has this cause occurred in production? │
│ Review field failure data / complaints │
└─────────────────────────────────────────┘
RARELY → Rating: 7-8 (High)
OFTEN → Rating: 9-10 (Very High)
Severity × Occurrence × Detection Integration
PFMEA Occurrence feeds directly into Risk Priority Number (RPN) calculation, which combines three independent risk factors:
| Factor | Scale | Represents |
|---|
| Severity (pfmSeverity) | 1-10 | Impact if cause occurs — NOT affected by controls |
| Occurrence (pfmOccurrence) | 1-10 | Frequency cause occurs — REDUCED by prevention controls |
| Detection (pfmDetection) | 1-10 | Ability to detect cause — IMPROVED by detection controls |
Pre-Mitigation RPN = Severity × Pre-Occurrence × Pre-Detection
Post-Mitigation RPN = Severity × Post-Occurrence × Post-Detection
Severity typically does NOT change between pre- and post-mitigation because it represents the inherent impact of the failure. Only occurrence and detection should improve through control implementation. A severity rating should only decrease if the design itself is fundamentally changed.
AIAG-VDA FMEA Standards Alignment
The PFMEA Occurrence enumeration aligns with AIAG/VDA FMEA Handbook (2019) process failure mode assessment methodology:
- Prevention Controls directly reduce occurrence ratings by eliminating or containing root causes
- Detection Controls improve detection ratings but do NOT reduce occurrence (detection does not prevent failure)
- Action Priority (AP) classification thresholds are applied to post-mitigation RPN values to guide resource allocation
Recommended action thresholds:
- RPN 1-40: Low priority (document and monitor)
- RPN 41-100: Medium priority (plan and implement controls)
- RPN > 100: High priority (immediate action required)
Process Control Types and Occurrence Reduction
Prevention Controls (Reduce Occurrence)
Prevention controls eliminate or reduce the likelihood of the root cause occurring:
| Control Type | Effectiveness | Typical Occurrence Reduction |
|---|
| Error-proofing (Poka-yoke) | Highest | 1-2 rating (> 90% reduction) |
| Design modification | High | 2-3 rating (75-90% reduction) |
| Process parameter control | Moderate-High | 3-4 rating (50-75% reduction) |
| Operator training | Moderate | 5-6 rating (25-50% reduction) |
| Work instructions | Low-Moderate | 6-7 rating (10-25% reduction) |
| No control | Baseline | 8-10 rating (0% reduction) |
Detection Controls (Do NOT Reduce Occurrence)
Detection controls identify failures after they occur but before customer impact:
- 100% inspection / automated gauging
- Statistical sampling plans
- Visual inspection procedures
- In-process testing
A detection control (e.g., automated measurement) improves the Detection rating but leaves the Occurrence unchanged. Only prevention controls (e.g., fixture redesign, poka-yoke) reduce occurrence. Never confuse the two in FMEA assessment.
Workflow Integration
PFMEA Risksheet Columns
In the Process FMEA Risksheet, Occurrence columns appear in the structured hierarchy:
Level 1: Process Step (row header)
Level 2: Failure Mode (collapsible group)
└─ Level 3: Cause (detailed row)
├─ Cause Description
├─ Effect of Failure
├─ Severity (static)
├─ OCCURRENCE ← Pre-Mitigation (Set Ranking phase)
├─ Detection (Pre-Mitigation)
├─ RPN Pre = S × O × D
├─ Prevention Controls
├─ Detection Controls
├─ OCCURRENCE ← Post-Mitigation (Final Ranking phase)
├─ Detection (Post-Mitigation)
├─ RPN Post = S × O × D
└─ Action Priority (derived from RPN)
Traffic Light Validation
The Risksheet configuration includes traffic light rules that flag incomplete assessments:
trafficLights:
- field: "premitigationPFMOccurrence"
condition: "equals(0)"
color: "red"
message: "Occurrence rating required"
- field: "postmitigationPFMOccurrence"
condition: "status != 'CLOSED' && equals(0)"
color: "yellow"
message: "Post-mitigation occurrence pending"
Common Assessment Mistakes
Mistake 1: Confusing Occurrence with Detection
- ❌ Setting occurrence = 1 because “we have 100% inspection”
- ✓ Keep occurrence unchanged if only adding detection controls; reduce it only if adding prevention controls
Mistake 2: Lowering Severity Instead of Occurrence
- ❌ Reducing severity from 8 to 5 when adding detection controls
- ✓ Severity stays 8 (impact is same); only occurrence/detection change with controls
Mistake 3: Using Subjective Estimates Without Data
- ❌ Rating occurrence = 4 based on “gut feeling”
- ✓ Reference Cpk indices, SPC charts, defect histories, or field failure data
Mistake 4: Forgetting Post-Mitigation Assessment
- ❌ Planning controls but never updating post-mitigation occurrence/detection ratings
- ✓ Complete both pre- and post-mitigation assessments for each cause before marking PFMEA as complete
| Version | Date | Changes |
|---|
| 1.0 | 2026-02 | Initial PFMEA Occurrence enumeration documentation |
See also: PFMEA Risksheet Configuration | Create Process FMEA Document
