Examining Risk Priority Numbers in FMEA
The Risk Priority Number (RPN) methodology is a technique for analyzing the risk associated with potential problems identified during a Failure Mode and Effects Analysis (FMEA). This article presents a brief overview of the basic RPN method and then examines some additional and alternative ways to use RPN ratings to evaluate the risk associated with a product or process design and to prioritize problems for corrective action. Note that this article discusses RPNs calculated at the level of the potential causes of failure (Severity x Occurrence x Detection). However, there is a great deal of variation among FMEA practitioners as to the specific analysis procedure and some analyses may include alternative calculation methods.
of Risk Priority Numbers
Rating scales usually range from 1 to 5 or from 1 to 10, with the higher number representing the higher seriousness or risk. For example, on a ten point Occurrence scale, 10 indicates that the failure is very likely to occur and is worse than 1, which indicates that the failure is very unlikely to occur. The specific rating descriptions and criteria are defined by the organization or the analysis team to fit the products or processes that are being analyzed. As an example, Figure 1 shows a generic five point scale for Severity [Stamatis, 445].
Figure 1: Generic five point Severity scale
After the ratings have been assigned, the RPN for each issue is calculated by multiplying Severity x Occurrence x Detection.
The RPN value for each potential problem can then be used to compare the issues identified within the analysis. Typically, if the RPN falls within a pre-determined range, corrective action may be recommended or required to reduce the risk (i.e., to reduce the likelihood of occurrence, increase the likelihood of prior detection or, if possible, reduce the severity of the failure effect). When using this risk assessment technique, it is important to remember that RPN ratings are relative to a particular analysis (performed with a common set of rating scales and an analysis team that strives to make consistent rating assignments for all issues identified within the analysis). Therefore, an RPN in one analysis is comparable to other RPNs in the same analysis but it may not be comparable to RPNs in another analysis.
The rest of this article discusses related techniques that can be used in addition to or instead of the basic RPN method described here.
RPNs and Percent Reduction in RPN
For example, if the initial ratings for a potential problem are S = 7, O = 8 and D = 5 and the revised ratings are S = 7, O = 6 and D = 4, then the percent reduction in RPN from initial to revised is (280-168)/280, or 40%. This indicates that the organization was able to reduce the risk associated with the issue by 40% through the performance of the FMEA and the implementation of corrective actions.
The Occurrence/Severity matrix provides an additional or alternative way to use the rating scales to prioritize potential problems. This matrix displays the Occurrence scale vertically and the Severity scale horizontally. The points represent potential causes of failure and they are marked at the location where the Severity and Occurrence ratings intersect. The analysis team can then establish boundaries on the matrix to identify high, medium and low priorities. Figure 2 displays a matrix chart generated with ReliaSoft's Xfmea software. In this example, the Occurrence and Detection ratings were set based on a ten point scale, the high priority issues are identified with a red triangle (up), the medium priority issues are identified with a yellow circle and the low priority issues are identified with a green triangle (down). Within the software, when the user clicks a point in the matrix, the description of the potential problem is displayed. For presentation in other documents, a text legend can be used to accompany the matrix graphic.
Figure 2: Occurrence/Severity Matrix generated with Xfmea's Plot Viewer
Issues by Severity, Occurrence or Detection
Figure 3 presents a graphical view of failure causes ranked by likelihood of occurrence in a pareto (bar) chart generated by Xfmea. This chart provides the ability to click a bar to display the issue description and to generate a detailed legend for print-ready output. Xfmea also provides this information in a print-ready tabular format and generates similar charts and reports for Severity and Detection ratings.
Figure 4: Sample risk ranking table
The letters and numbers inside the table indicate whether a corrective action is required for each case.
For example, according to the risk ranking table in Figure 4, if Severity = 6 and Occurrence = 5, then corrective action is required if Detection = 4 or higher. If Severity = 9 or 10, then corrective action is always required. If Occurrence = 1 and Severity = 8 or lower, then corrective action is never required, and so on.
Other variations of this decision-making table are possible and the appropriate table will be determined by the organization or analysis team based on the characteristics of the product or process being analyzed and other organizational factors, such as budget, customer requirements, applicable legal regulations, etc.
Within Xfmea, users have the option to roll up the RPN from the cause level to the effect, failure, function and/or item levels of the analysis. Figure 5 shows these calculated values in the hierarchical view of the analysis within Xfmea.
Figure 5: Xfmea project with higher level RPNs calculated.
Crowe, Dana and Alec Feinberg, Design for Reliability, Chapter 12 "Failure Modes and Effects Analysis." CRC Press, Boca Raton, FL, 2001.
McCollin, Chris, "Working Around Failure." Manufacturing Engineer, February 1999. Pages 37-40.
Stamatis, D.H., Failure Mode and Effect Analysis: FMEA from Theory to Execution. American Society for Quality (ASQ), Milwaukee, Wisconsin, 1995.