Reliability can be defined as “the probability that an item will perform its intended functions for a specified interval under stated conditions”. It can be measured by Mean Time Between Failures (MTBF) and predicted Reliability (R) = e-?t, where t is the time in service.
Availability can be defined as “the probability that an item is in an operable or committable state…” Availability is generally calculated as uptime/uptime+downtime or MTBF/MTBF+MDT where MDT = mean downtime.
So, what is the availability and reliability of a system over an 8 hour period, where the required operating period is 8 hours, and the system is up for 2 hours,
down for 1, back up for 2 hours, down for another hour, then up for 2 hours?
Answer: Availability = 6/6+2 = 75% ; Reliability = e 0.333 x 8 = .69 or 70%.
What is the significance? Watch for my next posting…