Risk Priority Number (RPN) - RPN is calculated by multiplying Severity (S), Occurrence (O), and Detection Ratings (D) for a FMECA
An integer Test limit from 1 to 1000 reflecting the qualitative risk for a given failure (with 1000 being the highest possible risk). This Test limit is calculated by multiplying the Severity, Occurrence and Detection Ratings for the given failure. The highest RPNs in the chart will reflect the most severe, most frequent, yet least detectable failures. Conversely, lower RPNs will be assigned to failures that are less severe, less frequent and/or more detectable.
This industry standard for Calculating risk of failure has been made flexible in eXpress to allow analyst customizing the method of Risk assessment utilizing the "Risk Priority Number (RPN)" calulations.
Where:
Severity: a rating of the adverse impact of the defect – a measure that reflects the negative consequence to the users or developers. eXpress has 3 methods for Calulating the Test limit (1 through 10) of Severity for use in the RPN calculation:
Occurrence: how often the defect is encountered and/or how long it takes to recover functionality – a measure that reflects a different element of the impact of the defect. eXpress has 3 methods utilized for Calulating the Test limit (1 through 10) of Occurrence for use in the RPN calculation:
Detection: how easy it is to spot the defect is when it occurs – a measure that reflects the risk of unmitigated consequences if the defect is not remedied. eXpress has 3 methods utilized for Calulating the Test limit (1 through 10) of Detection for use in the RPN calculation:
The major difference in eXpress between the Inductry RPN definitions is how the Detection Ratings are derived in eXpress. RPN calculations take into account the probability that a failure will be detected in a given mission phase. Note: Detection Ratings Relies on Diagnostic results for FMECA. Therefore Diagnostics must be included in the FMECA
Example: Failures with the same severity and occurence that are tested continuously would have a much lower RPN number than failures the are only tested at start up only or not tested for at all.Detection ratings derived for RPN Test limits in eXpress will distinguish these two possibilties from each other. "Risk is much lower with adequate testing." eXpress RPN calculations can show this so a determination can be made to increase reliabilty or testing for very high RPN assessments.
