FMEA Ratings Guide

Detailed guidance on assigning Severity, Occurrence, and Detection ratings.

Severity (S)

How severe is the effect on the customer? (1-10 scale)

Rating Table

Rating	Effect	Criteria
10	Hazardous without warning	Death or serious injury, no warning
9	Hazardous with warning	Death or serious injury, with warning
8	Very high	Vehicle/product inoperable, loss of primary function
7	High	Vehicle/product operable but reduced, customer dissatisfied
6	Moderate	Vehicle/product operable but comfort/convenience degraded
5	Low	Fit and finish/squeak and rattle item, noticed by most customers
4	Very low	Fit and finish/squeak and rattle item, noticed by discriminating customers
3	Minor	Fit and finish/squeak and rattle item, noticed by very few customers
2	Very minor	No effect noticed by customer
1	None	No effect

Important Notes

• Severity cannot be reduced through controls or testing
• Only design changes can reduce severity
• Always rate worst-case scenario
• Customer perspective only

Occurrence (O)

How frequently does the cause occur? (1-10 scale)

Rating Table

Rating	Probability	Cpk	Failure Rate
10	Very high: Failure almost inevitable	< 0.55	> 1 in 2
9	High: Repeated failures	0.55	1 in 3
8	High: Frequent failures	0.78	1 in 8
7	Moderate: Occasional failures	0.86	1 in 20
6	Moderate: Occasional failures	0.94	1 in 80
5	Low: Relatively few failures	1.00	1 in 400
4	Low: Relatively few failures	1.10	1 in 2,000
3	Very low: Isolated failures	1.20	1 in 15,000
2	Remote: Failure unlikely	1.30	1 in 150,000
1	Nearly impossible	1.67	< 1 in 1,500,000

How to Estimate

• Use data when available (test results, field data, warranty claims)
• Consider process capability (Cpk)
• Account for current design and controls
• Be realistic, not optimistic

Detection (D)

How likely are current controls to detect the failure before delivery? (1-10 scale)

Rating Table

Rating	Detection	Likelihood	Criteria
10	Absolute uncertainty	No chance	No inspection/control
9	Very remote	Very low	Random inspection only
8	Remote	Low	Visual inspection, manual
7	Very low	Moderate	Double visual inspection
6	Low	Moderately high	Manual gauging/measurement
5	Moderate	High	SPC with immediate reaction
4	Moderately high	Very high	SPC with reaction plan
3	High	Very high	Automated gauging, 100% inspection
2	Very high	Almost certain	Error-proofing, automated 100% inspection
1	Almost certain	Certain	Defect cannot be made (design prevents it)

Important Considerations

• Rate detection before customer receives product
• Consider all test and inspection methods
• Lower is better (easier to detect)
• Don't assume future improvements

RPN Calculation

RPN = Severity × Occurrence × Detection

RPN Priority Matrix

RPN Range	Priority Level	Action Required
125-1000	Critical	Immediate corrective action required
50-124	High	Action plan required with specific timeline
10-49	Medium	Action recommended, monitor closely
1-9	Low	No action required, continue monitoring

Mitigation Strategy

To reduce RPN, focus on:

1. Occurrence (easiest to reduce)

   • Improve design robustness
   • Add redundancy
   • Prevent root cause

2. Detection (moderate difficulty)

   • Add automated inspection
   • Improve test coverage
   • Error-proofing

3. Severity (hardest to reduce)

   • Redesign to eliminate/reduce effect
   • Add safety features
   • Change architecture

Focus Areas

A reduction in any factor reduces RPN. For example:

• S=10, O=6, D=7 → RPN=420
• Reduce O to 2: RPN=140 (67% reduction!)
• Reduce D to 3: RPN=60 (86% reduction!)

Common Mistakes

Over-Rating

❌ Don't:

• Rate every effect as severity 10
• Assume worst-case for occurrence
• Ignore existing controls

✅ Do:

• Use rating tables consistently
• Base on data when possible
• Consider current design state

Under-Rating

❌ Don't:

• Underestimate severity to avoid action
• Assume controls work perfectly
• Rate based on desired state

✅ Do:

• Be honest about risks
• Challenge assumptions
• Focus on facts

Examples

Example 1: Battery Overheat

• Severity: 10 (fire hazard, injury risk)
• Occurrence: 4 (rare, 1 in 500 charges)
• Detection: 6 (temperature monitoring, but not fail-safe)
• RPN: 240 (CRITICAL)

Action: Add automatic charging cutoff → O=2, D=3 → RPN=60

Example 2: Weld Defect

• Severity: 7 (product recall, safety risk)
• Occurrence: 5 (moderate, 1 in 100)
• Detection: 8 (visual spot check only)
• RPN: 280 (CRITICAL)

Action: 100% ultrasonic testing → D=3 → RPN=105 (still high, improve process)

What's Next?

• Creating FMEA - Apply ratings to your FMEA
• Actions and Mitigation - Reduce high RPNs