Guest Column | April 27, 2020

Is It Time To Say Goodbye To FMEA Risk Priority Number (RPN) Scores?

By Mark Durivage, Quality Systems Compliance LLC

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Failure mode effect analysis (FMEA) risk priority number (RPN) scores have traditionally been used to quantify risks for users, designs, and processes.

FMEA helps quantify and prioritize risk using criticality/severity, occurrence, and detection scores that are multiplied to produce the RPN. Criticality/severity (S) is a measure of the seriousness of the possible consequences of a hazard. Probability of occurrence (O) is the likelihood that a cause will occur and that it will induce the detrimental consequences of the associated hazard. Detection (D) is the probability that control (design, in-process, inspection, alert/warning) will eliminate, mitigate, or catch the defect. The RPN output of an FMEA is a relative risk score for each failure mode, which is used to rank the failure modes on a relative risk basis.

The scoring system typically used for severity/criticality, probability of occurrence, and probability of detection is generally from 1 to 5, with 1 considered low risk and 5 high risk. The RPN score is calculated by multiplying the severity/criticality, probability of occurrence, and probability of detection.

Example: A process FMEA has been developed for a new product using Table 1. A certain characteristic has a criticality/severity of 4, a probability of occurrence of 3, and a probability of detection of 3. The RPN is calculated as follows:

RPN = Severity x Occurrence x Detection = 4 x 3 x 3= 36

Table 1: Example Criticality/Severity, Occurrence, and Detection Ratings

Criticality/Severity

Definition

5

Catastrophic

4

Critical

3

Serious

2

Minor

1

Negligible

Occurrence

5

Certain to fail; 1 failure / hour

4

High number of failures; 1 failure / day

3

Occasional failure; 1 failure / week

2

Few failures likely; 1 failure / month

1

Failure unlikely; 1 failure / year

Detection

5

Undetectable

4

Poor

3

Moderate

2

Good

1

Excellent

 

According to Table 2, an RPN of 36 is considered undesirable. However, the severity and probability of occurrence should also be evaluated for this characteristic. Table 3 indicates for a criticality/severity of 4 with a probability of occurrence of 3, the risk is considered unacceptable.

With the current use of RPN scores, both Table 2 and Table 3 must be referenced to ensure the organization takes appropriate action.

Table 2: Example RPN Action Criteria

RPN

Risk Acceptability

Action

1 to 14

Low

Although low risk, continue mitigation process as far as possible

15 to 29

Tolerable

This should only be revisited in future design review if corrective action enhances the reliability or product appeal.

30 to 49

Undesirable

Risk is acceptable only if it cannot be further mitigated by organizational or technological solutions which do not reduce the clinical/functional utility of the product.

Above 49

Intolerable

Risk should be eliminated or reduced by protective measures. Justification required for risk that is accepted.

 

Table 3: Example Criticality/Severity and Occurrence Action Requirements

 

Criticality/Severity

Catastrophic

Critical

Serious

Minor

Negligible

Occurrence

 

5

4

3

2

1

Certain

5

Unacceptable

Unacceptable

Unacceptable

Unacceptable

ALARP

High

4

Unacceptable

Unacceptable

Unacceptable

ALARP

ALARP

Occasional

3

Unacceptable

Unacceptable

ALARP

ALARP

ALARP

Few

2

Unacceptable

ALARP

ALARP

ALARP

Acceptable

Unlikely

1

ALARP

ALARP

ALARP

Acceptable

Acceptable

*ALARP = As Low as Reasonably Possible

An Alternative Method

The Automotive Industry Action Group (AIAG) and the Verband der Automobilindustrie (VDA) jointly released an updated FMEA handbook on April 2, 2019. One of the major changes with the new AIAG-VDA FMEA process is the use of the RPN has been eliminated. The RPN has been replaced by an action priority (AP) table.

The AP tables assign one of three suggested rankings for each action based upon the S, O, and D values. The AP rankings are as follows:

  • High Priority. The FMEA team must identify appropriate actions or improve the prevention or detection controls or reduce the probability of occurrence.
  • Medium Priority. The FMEA team should identify an appropriate action or improve the prevention or detection controls or reduce the probability of occurrence.
  • Low Priority. The FMEA team could improve upon the prevention and detection rankings or reduce the probability of occurrence.

Example: A process FMEA has been developed for a new product using Table 4. A certain characteristic has a criticality/severity of moderate, a probability of occurrence of moderate, and a probability of detection of adequate.

Table 4: Example Criticality/Severity, Occurrence, and Detection Ratings

Criticality/Severity

Definition

Catastrophic

Moderate

Minor

Occurrence

Certain. 1 failure / hour

Moderate. 1 failure / week

Remote. 1 failure / year

Detection

Slight

Adequate

Excellent


Using Table 5, a criticality/severity of moderate, a probability of occurrence of moderate, and a probability of detection of adequate produce an Action Priority of high. This requires the FMEA team to identify appropriate actions or improve the prevention or detection controls or reduce the probability of occurrence.

Table 5: Action Priority Ratings

Severity

Criticality

Occurrence

Detection

Action

Priority

Catastrophic

Certain

Slight

High

Adequate

High

Excellent

High

Moderate

Slight

High

Adequate

High

Excellent

High

Remote

Slight

High

Adequate

High

Excellent

Medium

Moderate

Certain

Slight

High

Adequate

High

Excellent

Medium

Moderate

Slight

High

Adequate

High

Excellent

Medium

Remote

Slight

Medium

Adequate

Medium

Excellent

Low

Minor

Certain

Slight

Medium

Adequate

Medium

Excellent

Low

Moderate

Slight

Low

Adequate

Low

Excellent

Low

Remote

Slight

Low

Adequate

Low

Excellent

Low

 


Conclusion

It may be time to consider eliminating the use of the traditional RPN score and transitioning to the use of AP ratings. AP ratings are much simpler to use, do not require a calculation (eliminating the validation of a spreadsheet), and provides a single simple table reference to determine the appropriate level of action.

I cannot emphasize enough the importance of establishing procedures (documenting) to manage the tools and methods used. Best practice includes providing rationale for your organization’s use of risk management tools and activities. The requirements and risk management tools presented in this article can and should be utilized based upon industry practice, guidance documents, and regulatory requirements.

Reference:

  • FMEA Handbook 1st Ed, 2019, Automotive Industry Group (AIAG) and the Verband der Automobilindustrie (VDA), Southfield, MI.

About the Author:

Mark Allen Durivage has worked as a practitioner, educator, consultant, and author. He is managing principal consultant at Quality Systems Compliance LLC, an ASQ Fellow, and an SRE Fellow. He earned a BAS in computer aided machining from Siena Heights University and an MS in quality management from Eastern Michigan University. He holds several certifications, including CRE, CQE, CQA, CSQP, CSSBB, RAC (Global), and CTBS. He has written several books available through ASQ Quality Press, published articles in Quality Progress, and is a frequent contributor to Life Science Connect. Durivage resides in Lambertville, Michigan. Please feel free to email him at mark.durivage@qscompliance.com or connect with him on LinkedIn.