PFMEA Core Steps: Assess Risk, Assign Ratings, Calculate RPN

Step 4 – Assign Severity, Occurrence, and Detection Ratings

In this step of PFMEA, we evaluate each identified failure mode using three key rating scales:

Severity (S)

Measures how serious the consequence of the failure would be if it occurs.

• Scale: 1 (No effect) to 10 (Hazardous)

• Example: If a brake system failure can lead to an accident, severity = 10

Occurrence (O)

Estimates how frequently a specific failure is likely to occur.

• Scale: 1 (Rarely happens) to 10 (Happens frequently)

• Example: A loose bolt falling off once every 10,000 cycles = 3

Detection (D)

Measures the ability of current controls to detect the failure before it reaches the customer.

• Scale: 1 (Very likely to be detected) to 10 (Almost impossible to detect)

• Example: If a visual inspection will always catch the defect = 2

Common Mistakes in Rating Assessments:

• Overestimating detection ability – Teams often assume detection is perfect, which lowers actual risk.

• Using biased opinions – Ratings should be based on data, not guesses or assumptions.

• Not updating ratings – Once controls improve, ratings must be revised accordingly.

Step 5 – Calculate the Risk Priority Number (RPN)

The RPN is the key number that helps prioritize which failures need urgent action.

RPN Formula:

$$\text{RPN}=\text{Severity (S)}\times \text{Occurrence (O)}\times \text{Detection (D)}$$

Example Calculation:

• Severity = 8

• Occurrence = 5

• Detection = 7

$$\text{RPN}=8\times 5\times 7=280$$

Thresholds for Action:

• RPN < 100: Low Risk — monitor

• RPN 100–200: Medium Risk — plan for control

• RPN > 200: High Risk — needs immediate corrective action

Case Study: Risk Prioritization

Case: Food packaging line failure due to sealing error

• Severity = 7 (potential food spoilage)

• Occurrence = 6 (1 in 500 items)

• Detection = 8 (no inspection in place)

RPN = 7 × 6 × 8 = 336 → High Priority

Action: Add automated vision inspection and operator training.