Which statement best defines reliability?

• A. Reliability is the probability of operating without failure for a specified period.
• B. Reliability is the fraction of time the system is operational.
• C. Reliability is the mean time between failures.
• D. Reliability is the time to repair after failure.

Answer: (A)

Reliability is the probability that a system operates without failure for a specified period. This is captured by the reliability function R(t) = P(T > t), the chance that the time to failure T exceeds the mission time t. So a statement that the system will work without failure for a given duration directly defines reliability. The other ideas describe related but different concepts: the fraction of time the system is operational is availability, which includes uptime and downtime; mean time between failures is MTBF, the average interval between failures rather than the likelihood of surviving a mission; and the time to repair after a failure is the repair time, which measures how long repairs take rather than the probability of operating without failure. Therefore, the best definition is the probability of operating without failure over the specified period.