Integer attribute (defined for failure effects) containing the desired occurrence rating (manually entered or imported for each object effect). When included in a FMECA, the values from this column will be used when calculating the Risk Priority Number (RPN). Risk Priority and Criticality Calculations in FMECA
Similar to the severity rating, the occurrence rating can be included in the FMECA by adding the “Occurrence Rating (for RPN)” column to the list of selected columns on the FMECA Plus Options dialog.
In the example at left, the occurrence rating will not only be listed in its own column, but will also be used when calculating the value in the “RPN (Risk Priority Number)” column.
The following excerpt shows how the occurrence rating would be calculated if the occurrence time interval were set to 20,000 hours for the entire system (meaning that the occurrence ratings are based on the likelihood of a failure occurring within a useful life of 20,000 system time):
If we were to increase the occurrence time interval (in this example, system useful life) to 50,000 hours, we would expect the occurrence rating to increase—even though the failure rate has not changed. This reflects the fact that, the longer the system is in use, the more likely failures are to occur.
Both the calculated occurrence rating and RPN are indeed larger. Notice that, even though the system is being fielded for two-and-a-half times as long as before (50,000 hours instead of 20,000 hours), the occurrence ratings only increased by one (since the mapping is not linear). Notice that the proportions between the calculated RPN numbers are different as well (for the same reason).
