To calculate MTB results for a system tree configuration, at least one record must be inserted in the FRACAS Operating Times table for the configuration.

Operating time records establish the time basis for the collection of failure data. Because operating timing information is based on a physical representation of the system, the operating time record for the configuration is applied to all items in that configuration.

For example, if you are recording problems with a computer system, you insert an operating time record for that particular configuration. The operating time record and the incidents for this configuration establish the basis for MTB (mean time between) calculations.

Operating time is basically the difference between a start time and end time, with adjustments for duty cycle. For example, assume that one incident (failure) is recorded for a computer system that has been in operation for 8 hours a day for 100 days. The MTBF (mean time between failures) is equal to 800 hours. If two incidents had been recorded for this system, the MTBF is equal to 400 hours.