Reliability

Reliability concerns correct operation over a specified interval and set of conditions. Availability asks how often the service is usable. The distinction matters because frequent short interruptions and rare long outages can have different effects even when their total downtime is similar.

What is reliability?

Reliability is the probability that a service performs its intended functions for a specified period under stated conditions. Define the function, conditions, and interval before comparing two reliability claims.

Mean time between failures (MTBF) and mean time to repair (MTTR) summarize failure and recovery observations. They help describe reliability and availability, but an average alone does not describe the distribution or impact of incidents.

With the definitions used in these equations, MTBF counts operating time per observed failure, while MTTR counts repair time per repair. Use consistent units and clarify what is included in maintenance time. Designers generally seek a high MTBF and a low MTTR.