FMEA RPN has widely known as a Risk Priority Number (indicator) in an FMEA. But how to calculate it quickly and evaluate the FMEA RPN effectively? Let’s deep dive in!
1. Risk Priority Number definition
Risk Priority Number is a numerical assessment of the risk priority level of a failure mode/failure cause in an FMEA analysis. It helps the responsible team/individual prioritize risks and decide on the corrective actions.
2. How to calculate the Risk Priority Number?
FMEA RPN is calculated by multiplying Severity (S), Occurrence (O) Or Probability (P), and Detection (D) indexes. Severity, Occurrence, and Detection indexes are derived from the failure mode and effects analysis:
Severity: The severity of the failure mode is rated on a scale from 1 to 10. A high severity rating indicates severe risk.
Occurrence (or Probability): The potential of failure occurrence is rated on a scale from 1 to 10. A high occurrence rating reflects high failure occurrence potential.
Detection: The capability of failure detection is rated on a scale from 1 to 10. A high detection rating reflects low detection capability.
For example: from the Process FMEA of a painting process, the RPN is 120 for the failure mode of foreign body in the painting layer.
Two options for calculating RPN in an FMEA worksheet
Method 1: Use the Excel formula. Because of the structure of the FMEA worksheet, the Severity cell is not linked one to one with the Occurrence and Detection cell, and the RPN formula needs to be created manually one by one.
Method 2: Use FMEA Studio Add-in inside Excel: Create a column with RPN column type inside Column Properties. RPN will be calculated automatically within a second for the whole FMEA worksheet. More detail about this feature are here.
RPN is not the only risk assessment number used with FMEA. Some companies use other indexes to assess risks, such as the Critical Number (CN) or Severity-Occurrence-Detection (SOD). However, they are rarely used.
2.1 Critical Number (CN)
Some companies find it difficult to decide on a detection ranking. Therefore, they do not use detection (D) in their calculation of the RPN to avoid arguments regarding the detection ranking.
The FMEA RPN does not cover all the numbers from 1 to 1000. Therefore, some companies use the numerical value of SOD. In this case, the ratings of S, O, and D range from 0 to 9 (not from 1 to 10).
Consider the following example.
|Failure Mode 1||8||1||1||811|
|Failure Mode 2||7||9||9||799|
Although Failure Mode 1 has a higher SOD than Failure Mode 2, it has lower risk because its potential for occurrence is small, and the possibility of detection is high (if it happens).
Thus, the disadvantage of SOD is that severity has a more critical role than Occurrence and Detection in the SOD equation.
3. How to evaluate Risk Priority Number?
The RPN can be used to prioritize high-risk issues and determine the requirement for corrective action. After calculation, most companies prioritize risks from the highest to the lowest RPN.
The team can use the Risk Priority Number to prioritize and reduce risks in the two following methods:
3.1 FMEA RPN Threshold
Many organizations use an RPN limit to determine which failure mode requires corrective action and which risks are acceptable. The RPN threshold is easy to use.
However, using an RPN threshold may cause team members to spend excessive time trying to reduce the Detection, Occurrence, and Severity rankings for lowering the RPN. This situation sometimes places the organization and its customers in danger.
3.2 Top RPNs
The other organizations may address the corrective action for the top failure modes/failures causes by Risk Priority Number. After that, the team will work with other top RPNs to continuously improve the process. This loop will continue until the quality target is met.
4. Risk Priority Number’s disadvantage
The two ways above are based only on the comparison between the RPN of failure modes. The comparison based on the RPN may not reflect the actual risk. A high RPN failure does not necessarily indicate a high risk for the process or product. Moreover, two failure modes with the same RPN value may not have the same risk level.
In the following example from the automotive industry, Failure Mode 1 should be provided a higher priority than Failure Mode 2 although they have the same RPN value because it has a higher Severity value.
|Failure Mode 1||7||4||4||112|
|Failure Mode 2||4||4||7||112|
However, because they have the same RPN value, an inappropriate action plan for critical failure may be selected. For this reason, the RPN should not be the only index used to evaluate the risk of each failure mode. The team should also use Severity, Occurrence, and Detection to prioritize risks.
5. How to evaluate the Risk Priority Number effectively?
5.1 Risk evaluation using the FMEA RPN with the consideration of the S, O, and D rankings
Along with RPN, the team should consider the S, O, and D rankings for prioritizing risks one by one through team discussion. However, the team will be overloaded if there are too many failure modes in the FMEA.
Moreover, teams can go through unending arguments to determine which failure requires corrective action.
5.2 Risk Matrix (Priority Matrix)
The team can combine the criteria for the RPN and Severity, Occurrence, and Detection rankings by using a matrix. The following is an example of a risk matrix for the RPN and Severity ranking.
The following example of a risk matrix includes a combination of severity and occurrence.
• Red: must eliminate the risk or reduce its level.
• Yellow: should eliminate the risk or reduce its level but not mandatory.
• Green: Acceptable risk.
The team can use a three-dimensional matrix of Severity, Occurrence, and Detection. However, this type of risk matrix is complicated to use without an FMEA software like FMEA Studio.
The organizations can create a matrix depending on their process and available resources. However, the team should apply the criteria consistently.
Using the RPN in FMEA is the most common and convenient method of evaluating risk because the FMEA RPN is easy to understand, whereas the entire FMEA methodology is complicated.
However, using only the RPN to determine the failure mode that requires corrective action can cause severe quality problems in an organization.
Companies that wish to avoid the limitations of the RPN should evaluate risk with a combination of indices, such as Severity, Occurrence, and Detection.