The reason for this has to do with the value of the risk assessment. Most FMEA standards, such as AIAG, SAE J1739 and VDA, attribute the occurrence ranking to the cause of the failure. This question has been debated in the FMEA community. Is it the occurrence of the failure mode or the cause? If the assessment of occurrence ranking falls between discreet occurrence numbers on the scale (such as between 4 and 5) the team should use the higher number. However, the FMEA team should endeavor to be as objective as possible, using the criteria from the occurrence scale to help determine the appropriate rank. There will always be a subjective element to this ranking, as the FMEA is done on new designs, design changes, and/or new applications. This assessment of occurrence ranking should be as objective as possible, using past field history of similar items, previous test results, experience with similar systems, and other sources of information. The team determines the likelihood of the failure mode / cause based on the above definition, and using the agreed upon occurrence scale, carefully reviews the criteria column to establish the occurrence ranking. How is Occurrence risk assessed in FMEAs? The occurrence ranking has a relative meaning rather than an absolute value and is determined without regard to the severity or likelihood of detection. It is based on the criteria from the corresponding occurrence scale. For Process FMEAs the occurrence ranking considers the likelihood of occurrence during production. For System and Design FMEAs, the occurrence ranking considers the likelihood of occurrence during the design life of the product. “Occurrence” is a ranking number associated with the likelihood that the failure mode and its associated cause will be present in the item being analyzed. The Oxford English dictionary defines “occurrence” as “the fact or frequency of something happening.” What is the definition of “occurrence” in an FMEA? Jorge Luis Borges Definition of “occurrence” “Reality is not always probable, or likely.” Still others connect associate it with Cause? Which is correct? This article discusses occurrence risk, including examples, and answers this question. Some FMEA teams associate occurrence ranking with Failure Mode others associate it with Effect.
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