Reliability is the probability that an item will perform its required function without failure under stated conditions for a stated period of time. When calculating reliability, the important factor is time. The calculation of reliability takes into account a mission time, which is the operational time for the item.

Reliability is expressed as a probability value, with values ranging from 0 to 1. A reliability value of 0 means that the system always fails within its mission time. A reliability value of 1 means that the system never fails within its mission time.

Assume that the MTBF calculated for an item is 500,000 hours. This means that the item is expected to operate approximately 500,000 hours before failure. If the mission time for this item is 100 hours, the probability that it would operate throughout this mission would be very high. If, however, the MTBF for the item was only 50 hours, the probability that it would continue to operate throughout a 100-hour mission would be very low.

For items with constant failure rates, the equation for calculating reliability is: R = e−λt

Where t is the mission time and λ is the failure rate in failures per hour.

The mission time is specified in the Project’s Mission Profile file. For more information, see Mission Profile Files.