Learn in the textbookReliability Metrics
DefinitionAvailability (稼働率) is the share of the total operating time in which the system worked normally, calculated as MTBF ÷ (MTBF + MTTR).
ExplanationIt is expressed as a value from 0 to 1 (or a percentage), and the closer it is to 1, the more available the system is in the broader sense (可用性): usable whenever it is needed. Lengthening MTBF or shortening MTTR raises it. With devices in series (直列) the system runs only while every device is running, while in parallel (並列) it runs as long as any one device is running.
AnalogyIt is the share of days in a year on which your bike was in good enough shape to ride.
Memory hookThe formula has the shape B ÷ (B + R): the numerator is just the B of MTBF, and the denominator adds the R of repair to make up the whole time.