The Problem with Risk Priority Numbers

Almost everyone who uses FMEAs is taught to use the RPN to determine on which line of the FMEA to work. Customers tell their suppliers they want recommended actions written for all lines with RPNs over a certain value. Auditors ask auditees for the RPN level that triggers the need for a recommended action. The following example demonstrates the problems with this approach.

 Figure 1: Two sets of ratings from two lines of a process FMEA. Severity    Occurrence     Detection     Risk Priority #    8                    5                    1                     40   8                    1                  10                     80

Using the automotive industry's generally accepted rating definitions, the first line in Figure 1 indicates the following: The severity rating of 8 means that the unacceptable process output will result in a loss of product function. The occurrence rating of 5 indicates that the process failure cause for the line will produce the unacceptable output at a probability between 1/400 and 1/2,000. The detection rating of 1 indicates that inspection is available in the process to successfully sort out the defect.

The second line indicates the following: The severity rating of 8 means that the unacceptable process output will result in a loss of product function. The occurrence rating of 1 indicates that the process failure cause for the line will produce the unacceptable output at a probability less than 1/1,500,000. Since the probability of causing the defect is so rare, the operating personnel have chosen not to try to prevent the cause of the defect or inspect for it. Since there are no process controls available, the detection rating must be a 10.

A quick review of the RPNs shows that if an RPN of 50 (typical) was used as a guideline, recommended actions would be developed to either prevent a cause that rarely results in a defect or inspect for a defect that rarely happens. All this will be done while the process is rejecting between 1/400 to 1/2,000 of its products without any signs that action is needed. When users of FMEAs are guided by the document to work on items that are logically incorrect, they lose respect for the FMEA process.

Those who use FMEAs need to learn that the class value is more important than the RPN. The class value, derived from the severity/occurrence matrix shown in Figure 2 below, puts the RPN into its proper perspective. The class matrix shows that any FMEA line with a severity rating of nine or 10 lies within the Legal Zone and poses a possible legal risk. These must be addressed regardless of the occurrence rating. The Warranty Zone shows combinations of severity and occurrence ratings that could lead to field failure with a probability that is unacceptable to the company (typically > 1/15,000). These items also need to be addressed.

In the example in Figure 1, the RPN of 40 seems to indicate no need for action. However, the class matrix indicates that line one falls into the Warranty Zone, indicating a problem that will lead to dissatisfied customers and cost the company money.

Line two in Figure 1 shows an RPN of 80 that would raise flags for an auditor but doesn't fall into either the Legal Zone or Warranty Zone. Therefore, careful consideration must be given as to whether to spend time and money correcting this problem.

Unfortunately, auditors and those who use FMEAs often look only at the RPN and ignore the class, which leads to wasted effort on problems of little significance.

Figure 2: Class matrix

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