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Risk Priority Number (RPN)

1970
Risk assessment meeting with engineers analyzing Risk Priority Numbers in a professional office.

(generated image for illustration only)

The Risk Priority Number (RPN) is a quantitative measure used in FMEA to prioritize risks. It is calculated as the product of three ranked factors: Severity (S), Occurrence (O), and Detection (D). The formula is [latex]RPN = S times O times D[/latex]. Each factor is typically rated on a scale from 1 to 10, allowing teams to focus on the highest-scoring risks first.

The RPN provides a simple, numerical method to rank the potential failure modes identified during an FMEA session. The three components are critical to its calculation. ‘Severity’ (S) assesses the seriousness of the end effect of the failure, with 1 being ‘no effect’ and 10 being ‘catastrophic failure with potential fatality’. ‘Occurrence’ (O) estimates the likelihood that a specific cause will occur and result in the failure mode, where 1 is ‘extremely unlikely’ and 10 is ‘inevitable’. ‘Detection’ (D) evaluates the ability of current controls to detect the cause or the failure mode before it reaches the customer, with 1 being ‘certain detection’ and 10 being ‘cannot be detected’.

While powerful, the RPN has known limitations. A key criticism is that different combinations of S, O, and D can yield the same RPN, yet represent vastly different risk profiles (e.g., S=10, O=2, D=2 gives RPN=40, and S=4, O=5, D=2 also gives RPN=40). The first case is a high-severity issue that should likely be addressed regardless of its low RPN. This has led to the development of alternative prioritization methods, such as the Action Priority (AP) tables in the AIAG-VDA FMEA handbook, which prioritize actions based on combinations of S, O, and D rather than just their mathematical product.

UNESCO Nomenclature: 1209
– Statistics

Type

Software/Algorithm

Disruption

Substantial

Usage

Widespread Use

Precursors

  • failure mode and effects analysis (fmea) methodology
  • decision theory
  • probability theory
  • qualitative risk assessment techniques

Applications

  • prioritizing corrective actions in manufacturing
  • assessing risks in medical device design
  • evaluating software bug severity
  • managing project risks
  • environmental impact assessment

Patents:

NA

Potential Innovations Ideas

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Related to: RPN, risk priority number, FMEA, severity, occurrence, detection, risk assessment, quantitative analysis, quality engineering, risk matrix.

Historical Context

Risk Priority Number (RPN)

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1970-01-01
1975-06-01

(if date is unknown or not relevant, e.g. "fluid mechanics", a rounded estimation of its notable emergence is provided)

Related Invention, Innovation & Technical Principles

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