Six Sigma Certification: Risk Priority Number and Scale Crieteria - Occurence

Risk Priority Number or RPN is a measure used when assessing risk to help identify critical failure modes associated with a design or process. Just like we have capability indices to measure Process Capability, we have RPN as a measure of FMEA. The RPN values range from 1 (absolute best) to 1000 (absolute worst). So FMEA helps to know the worst failure scenario and then teams can work on it to either eliminate it or mitigate it. Higher the RPN, higher is the risk associated with that particular failure.

There is a term used to study the impact of failure on the process and design in an Organization.It is called as Severity. Severity is the seriousness of the effect of the failure mode. But how do you rate seriousness of a failure on a scale of 1 to 10? By using a severity table. Different industries will have different structure of severity table. We show you somewhat generalized table of severity. You can understand from this table that severe the failure, higher is its rating.
A team working on FMEA of a radioactive plant might want to put “fatal” as the effect with rating 10. As said severity table varies with industry and scenario. Lets take an example. A manager of a sports team wants to rate the severity of failure of the team in the next game. She might rate it at 9 given the fact that the team would lose a big sponsorship should they face defeat. This could be hazardous to the teams’ future.

Occurrence is the probability that a specific cause will result in the particular failure mode. As with severity, we rate occurrence on a scale of 1 to 10 based on a table. Like severity table, higher the occurrence of a failure, higher is its rating. Again, this table might vary depending on the industry and scenario. We present you a generalized table here. Sometimes, the project team can use data here if they have it. Based on past data, the probability of occurrence of a failure can easily be rated. But again, data is not necessary.
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