FMEA and Risk Priority Number (RPN)

Definition and Overview of Risk Priority Number (RPN)

The Risk Priority Number (RPN) is a crucial quantitative indicator used to evaluate risks in products and processes. It is calculated as the product of three fundamental elements:

Severity (S) × Occurrence (O) × Detection (D)

This calculation method is widely recognized as a practical approach that considers economic feasibility. Each element is typically rated on a scale from 1 to 10, resulting in an RPN ranging from 1 to 1,000.

The RPN serves as a prioritization tool, helping organizations allocate resources efficiently by focusing on the most critical risks first. However, it’s important to note that RPN should not be the sole criterion for decision-making, as recent standards emphasize a more holistic approach to risk assessment.

Risk Priority Evaluation Process

To properly evaluate risk priorities, a systematic approach is essential. The evaluation process is typically conducted in two main stages:

First Stage: Severity and occurrence probability are analyzed in detail and combined to create a risk matrix. At this stage, the basic classification of each risk is determined. This initial assessment helps identify which failure modes require immediate attention based on their potential impact and likelihood.

Second Stage: The risk classification obtained in the first stage is combined with detectability for evaluation. This determines the final risk priority, which forms the basis for deciding the implementation sequence of countermeasures. During this stage, teams should consider whether existing controls are adequate or if additional detection methods are needed.

It’s worth noting that modern risk management approaches, as reflected in standards such as ISO 14971 (medical devices) and IATF 16949 (automotive), increasingly emphasize that risk evaluation should not rely solely on RPN. Instead, organizations should consider each component (Severity, Occurrence, Detection) individually, particularly when severity is high, regardless of the overall RPN value.

How to Calculate Risk Priority Number (RPN)

The RPN calculation involves three key parameters:

Severity (S): Represents the seriousness of the effect of a potential failure mode on the customer or end-user. Severity ratings typically range from 1 (no effect) to 10 (hazardous without warning). This rating is determined based on the worst-case consequence of the failure and should consider regulatory requirements, safety implications, and customer impact.

Occurrence (O): Indicates the likelihood that a specific cause will occur and result in the failure mode. Occurrence ratings range from 1 (extremely unlikely) to 10 (inevitable). This rating should be based on historical data, statistical analysis, or expert judgment when data is unavailable.

Detection (D): Measures the ability of current controls to detect the cause of the failure mode or the failure mode itself before the product reaches the customer. Detection ratings range from 1 (almost certain detection) to 10 (absolute certainty of non-detection). Lower detection ratings indicate more effective control measures.

The RPN is then calculated by multiplying these three values:

RPN = S × O × D

For example, if a failure mode has a severity rating of 8, an occurrence rating of 4, and a detection rating of 6, the RPN would be: 8 × 4 × 6 = 192

Organizations typically establish threshold values (commonly 100-125) above which action is required. However, any failure mode with high severity (typically 9 or 10) should be addressed regardless of the RPN value.

Historical Development of FMEA

FMEA (Failure Mode and Effects Analysis) is a systematic methodology for analyzing potential failure modes of products or processes and evaluating their effects.

1940s – Military Origins: FMEA was first introduced by the United States military with the objective of improving reliability. The U.S. Armed Forces developed MIL-P-1629, titled “Procedures for Performing a Failure Mode, Effects and Criticality Analysis,” which became the foundation for modern FMEA practices.

1960s – Aerospace Application: The application expanded into the aerospace sector, where FMEA played a particularly important role in NASA’s Apollo program. The success of FMEA during this period led to widespread recognition of its effectiveness. The catastrophic failures in early space missions highlighted the need for systematic failure analysis, making FMEA an indispensable tool.

1970s – Automotive Industry Adoption: Ford Motor Company pioneered the introduction of FMEA into the automotive industry, establishing it as a standard quality management tool in manufacturing. Around the same time in Japan, FMEA began to be incorporated into product evaluation processes prior to mass production. This period marked the transition of FMEA from aerospace and defense applications to mainstream manufacturing.

1980s-1990s – Standardization: The automotive industry, through organizations like the Automotive Industry Action Group (AIAG), developed standardized FMEA procedures. This led to the creation of the FMEA reference manual, which has undergone several revisions.

2000s-Present – Evolution and Integration: A notable development is the Design Review Based on Failure Mode (DRBFM) developed by Toyota Motor Corporation. This evolved from conventional FMEA and was established as a methodology that specifically focuses on design changes and change points in manufacturing processes. DRBFM emphasizes the analysis of what has changed from a previous, proven design, making it particularly effective for incremental improvements and modifications.

Current Standards: The AIAG & VDA FMEA Handbook (published in 2019) represents the latest harmonization between American (AIAG) and German (VDA) automotive industry standards. This handbook introduced significant changes, including a shift from RPN-based prioritization to an Action Priority (AP) system, reflecting modern risk management philosophy.

Detailed Understanding of Failure Modes

A failure mode represents the manner in which a product or system could potentially fail. It is a systematic classification of possible failure types that might occur. Specific examples include mechanical wear, electrical disconnection, material degradation, software errors, and human errors in operation or assembly.

When practicing FMEA, it is essential to analyze in detail the various change points in each process. These elements include:

• Changes in equipment condition (e.g., wear, calibration drift, or replacement of components)
• Worker rotation and shifts (affecting skill levels and consistency)
• Changes in work methods or procedures (including process improvements or temporary workarounds)
• Changes in parts or materials used (including supplier changes or material substitutions)
• Changes in environmental conditions (temperature, humidity, contamination levels, or lighting)
• Changes in production volume or rate (affecting process stability)
• Introduction of new technology or automation
• Modifications to tooling or fixtures

For instance, when workers rotate on a manufacturing line, potential failure modes include quality variations due to differences in technical skill levels, familiarity with the process, or training adequacy. Similarly, when a supplier changes, even for an “equivalent” part, differences in dimensional tolerances, material properties, or manufacturing processes can introduce new failure modes.

The analysis of failure modes should not be limited to physical failures. It must comprehensively consider potential problems arising from human factors, process variations, and system interactions. This includes:

Human Factors: Fatigue, distraction, misunderstanding of procedures, communication failures, or inadequate training.

Process Factors: Material variability, equipment drift, environmental changes, or cascading effects from upstream processes.

System Factors: Software bugs, integration issues, electromagnetic interference, or unintended interactions between components.

By conducting such comprehensive analysis, it becomes possible to implement effective countermeasures aimed at improving product and process reliability. Modern FMEA practice encourages multidisciplinary team participation, bringing together expertise from design, manufacturing, quality, maintenance, and other relevant functions to ensure thorough identification of potential failure modes.

Integration with Quality Management Systems

FMEA is not a standalone tool but should be integrated into an organization’s overall quality management system. It connects with other quality tools such as:

• Control Plans: FMEA outputs directly inform the development of control plans, specifying which characteristics require monitoring and what control methods should be used.
• Process Flow Diagrams: These provide the foundation for Process FMEA by mapping out each step where failure modes could occur.
• Statistical Process Control (SPC): High-priority failure modes identified in FMEA often become candidates for SPC monitoring.
• Design Verification and Validation: Design FMEA findings guide the scope and focus of verification and validation activities.

Best Practices and Common Pitfalls

Best Practices:

• Conduct FMEA early in the design or process development phase when changes are less costly
• Use cross-functional teams to leverage diverse expertise and perspectives
• Base ratings on objective data whenever possible rather than subjective judgment
• Regularly review and update FMEAs as designs mature, processes change, or field data becomes available
• Document assumptions and rationale for ratings to ensure consistency and facilitate future reviews
• Focus on prevention rather than detection – it’s better to prevent failures than to catch them

Common Pitfalls to Avoid:

• Treating FMEA as a paperwork exercise rather than a genuine risk analysis tool
• Conducting FMEA too late in the development cycle when design changes are expensive
• Using overly optimistic ratings to artificially lower RPN values
• Failing to implement or verify the effectiveness of recommended actions
• Neglecting to update FMEA when changes occur
• Relying solely on RPN for prioritization without considering severity independently

Conclusion

FMEA, combined with RPN analysis, remains a fundamental tool for proactive risk management in product development and manufacturing. While the methodology has evolved significantly since its military origins, the core principle remains unchanged: systematically identify potential failures before they occur and implement effective preventive measures.

Modern practice emphasizes that risk assessment should be multifaceted, considering not just the numerical RPN but also the individual components of risk and the broader context of safety, regulatory compliance, and customer satisfaction. When implemented effectively as part of an integrated quality management system, FMEA contributes significantly to improving product reliability, reducing costs associated with failures, and enhancing customer satisfaction.

Organizations should view FMEA not as a compliance burden but as a valuable investment in understanding and mitigating risks, ultimately leading to more robust products and processes. As industries continue to evolve with increasing complexity and stricter regulatory requirements, the role of FMEA in ensuring quality and safety will only become more critical.