Understanding RPN Limitations – Problems and Solutions

In this week’s FMEA problems and solutions article, the intermediate problem challenges readers to prioritize a series of RPNs (with their corresponding S, O, and D). In the advanced problem, readers are asked to weigh in on a fictitious debate between advocates of traditional RPN, and advocates of criticality assessment, using only severity and occurrence.

If you haven’t yet read the article titled “Prioritizing risk for corrective actions in an FMEA – Know before you go!“, you can access it by clicking on the link.

Beginner’s Problem

In an FMEA, which of the following is true about “Risk Priority Number (RPN)”? (Select the best answer.)

1. An “RPN” is the sum of Severity, Occurrence, and Detection rankings.
2. An “RPN” is the product of Severity and Occurrence rankings.
3. An “RPN” is the product of Severity, Occurrence, and Detection rankings.
4. None of the above.

Beginner’s Solution

1. An “RPN” is the sum of Severity, Occurrence, and Detection rankings. (False. An “RPN” is the product of Severity, Occurrence, and Detection rankings, not the sum.)
2. An “RPN” is the product of Severity and Occurrence rankings. (False. An “RPN” is the product of Severity, Occurrence, and Detection rankings.)
3. An “RPN” is the product of Severity, Occurrence, and Detection rankings. (True)
4. None of the above. (False)

Intermediate Problem

You are performing an FMEA on a bicycle brake cable. The team has identified two failure modes for one of the primary functions, with two causes for each of the two failure modes (see illustration).

This results in four RPNs:
A) RPN 42  (S = 7, O = 3, D = 2)
B) RPN 140 (S = 7, O = 5, D = 4)
C) RPN 200 (S = 10, O = 5, D = 4)
D) RPN 40  (S = 10, O = 2, D = 2)

Using the letters A, B, C, D, what is the priority sequence for addressing issues in this FMEA excerpt?

Intermediate Solution

C, D, B, A
Recall the rule that FMEA teams should always address high severity first, regardless of RPN value. Therefore, the first priority is severity 10, RPN 200, which is letter C. The next priority is severity 10, RPN 40, which is letter D. Once the high severity issues area addressed, the team can take up high RPNs. The next priority is RPN 140, which is letter B. If the team wishes, it can finally address RPN 42, which is letter A.

Advanced Problem

A debate is raging in your company about the use of RPN. One side wants to adhere to the AIAG/SAE standards that recognize three risks: severity, occurrence and detection, and the resulting RPN value. The other side wants to limit risk characterization to severity and occurrence, instead using “criticality” (product of severity and occurrence).

Summarize briefly the pros and cons for both approaches.

Advanced Solution

There is no perfect answer to this problem. This debate has been going on for many years in the FMEA community. Use of RPN is predicated on the assumption that detection risk is sufficiently important that it needs to be addressed in FMEAs, and characterized in the risk priority number.

The argument in favor of including detection risk within FMEA risk prioritization goes something like this. Detection-type controls should be able to detect the failure mode/cause in order to ensure problems are not discovered by users; especially for higher-risk issues. Where detection-type controls are not able to detect the failure mode/cause, there is a potential for detection risk. FMEA teams can improve detection-type controls through the recommended action’s column, in order to reduce detection risk and ensure anticipated problems are discovered through testing and analysis during product development.

The argument against including detection risk within FMEA risk prioritization focuses on the classical definition of risk. The classical characterization of risk from ISO standards says, “Risk is often expressed in terms of a combination of the consequences of an event and the associated likelihood of occurrence.” The argument continues with the limitations to detection scales published by AIAG or SAE. Many consider the detection scales to be confusing and difficult to apply. Therefore, according to this argument, the use of “criticality” (SxO) is a better way to identify risk.

If RPN will be used, the company needs to understand the limitations of RPN and detection risk and act accordingly. For example, high severity must always be addressed regardless of RPN value. The company will need to develop a detection scale that makes sense, and detection-scale criteria verbiage that is clear and represents the varying risk due to likelihood of detection of failure modes and associated causes.

If “criticality” (SxO) is used, the company needs to understand the risk from lack of detection of failure modes/causes during product development, and find a way to address this risk. It is not acceptable for the end user to be the one to discover problems that were missed by testing, during the product development timeframe.

Next Article

Can you take into account reliability or durability functions in an FMEA? How can this be done? A reader asks this question, and it  is discussed and answered in the next FMEA Q and A article.

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