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Risk Priority Number (RPN) is a measure used when assessing risk to help
identify critical failure modes associated with your design or process. The RPN
values range from 1 (absolute best) to 1000 (absolute worst). The FMEA RPN is
commonly used in the automotive industry and it is somewhat similar to the criticality
numbers used in Mil-Std-1629A. The graphic below shows the factors that make up
the RPN and how it is calculated for each failure mode.
Severity (S) - Severity is a numerical subjective estimate of how severe
the customer (next user) or end user will perceive the EFFECT of a failure.
Examples of Severities.
Occurrence (O) - Occurrence or sometimes termed LIKELIHOOD, is a
numerical subjective estimate of the LIKELIHOOD that the cause of a
failure mode will occur during the design life, or during production in the
case of a Process FMEA. Examples of Occurrence values.
Detection (D) - Detection is sometimes termed EFFECTIVENESS. It is a
numerical subjective estimate of the effectiveness of the controls to prevent
or detect the cause or failure mode before the failure reaches the customer.
The assumption is that the cause has occurred. Examples of Detection
Some words of caution when using the RPN value to assess risk - RPNs have no
value or meaning in themselves. Although it is true that larger RPN values normally
indicate more critical failure modes, this is not always the case. For example, here
we have three cases where the RPNs are identical, but clearly the second case
would warrant the most attention.
As a general rule, any failure mode that has an effect resulting in a severity 9 or 10
would have top priority. Severity is given the most weight when assessing risk. Next,
the Severity and Occurrence (S x O) combination would be considered, since this in
effect, represents the criticality.
Below is another RPN example reminding us that we need to be careful not to assess
risk purely based on the RPN values. Here, the failure modes with the lowest RPN
values are actually the most critical. Be careful to not just establish "threshold values"
for RPNs when assessing risk, as this could lead you to make costly mistakes. Below
we see that #1 is most critical even though it has the lowest RPN value, then #2, and
In summary, always address high severity failure modes regardless of their
overall RPN values.