Materiel Reliability is a measure of the probability that the system will perform without failure over a specific interval. Reliability must be sufficient to support the warfighting capability needed. Materiel Reliability is generally expressed in terms of a mean time between failure(s) (MTBF), and once operational can be measured by dividing actual operating hours by the number of failures experienced during a specific interval.
Reliability may initially be expressed as a desired failure-free interval, which can be converted to MTBF for use as a KSA. (e.g. 95% probability of completing a 12 hour mission, free from mission-degrading failure; 90% probability of completing 5 sorties without failure, etc.) Specific criteria for defining operating hours and failure criteria must be provided together with the Key System Attribute (KSA). Single-shot systems and systems for which other units of measure are appropriate must provide supporting analysis and rationale.