Time attribute that specifies the time parameter of the probability of failure calculation (1 – e -λt). When the attribute is defined for objects, a separate t value will be used for each item. When defined at the design level, a single t value will be used for all probability of failure calculations. When omitted, the failure rate of each failure mode (rather than its probability of failure) is used when calculating occurrence ratings. Occurrence Map Checklist Checklist attribute (defined at the design level).
To define the occurrence time interval, create an attribute named “OccurrenceTimeInterval” and assign it the type Time.
If you wish to base each likelihood of failure on the useful life of the associated repair item, then associate this attribute with objects (with a default value greater than zero).
If, on the other hand, you wish to enter a single useful life value for the entire design, then associate this attribute with the design itself (as shown at left).
Note: When an occurrence time interval attribute is not defined, the failure mode rate (the failure ratio times the item failure rate) is used in place of the Q number.
When assigning values to this attribute, remember that any time value without units will be interpreted as hours. All values with units (minutes, days, etc.) will be automatically converted into hours when it is used to calculate the Q numbers (likelihoods of failure) that are used to determine the RPN occurrence rating. By default, the Q numbers calculated above will be mapped to a 10-point occurrence rating using the method documented in:
U.S. Army Technical Manual TM 5-609-4—Failure Modes, Effects and Criticality Analysis (FMECA) For Command, Control, Communications, Computer, Intelligence, Surveillance, And Reconnaissance (C4ISR) Facilities (09-29-2006).
The first two columns in the table at the right are copied directly from this document. The Q column has been added to help understand how values calculated using the equation above map to these rankings.
So, if a failure were to have at least a 1-in-20 chance of occurring during the specified useful life, but less than a 1- in-10 chance—a Q number between 0.05 (inclusive) and 0.1 (non-inclusive)—that failure would be assigned an 0.1 occurrence rating (aka ranking) of 9.
