unavailability

n. condition of not being available, unattainability, inaccessibility

Unavailability can be defined as the probability that an item will not operate correctly at a given time and under specified conditions. It opposes availability.Numerical values associated with the calculation of availability are often awkward, consisting of a series of 9s before reaching any significant numerical information (e.g. 0.9999999654). For this reason, it is more convenient to use the complement measure of availability, namely, unavailability. Expressed mathematically, unavailability is 1 minus the availability. Therefore, a system with availability 0.9999999654 is more concisely described as having an unavailability of 3.46E-8.
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Noun
1. the quality of not being unavailable when needed
(synonym) inaccessibility
(antonym) handiness, accessibility, availability, availableness
(hypernym) inconvenience

A expression of the degree to which a system, subsystem, or equipment is not operable and not in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., random, time. Note 1: The conditions determining operability and committability must be specified. (188 ) Note 2: Expressed mathematically, unavailability is 1 minus the availability. Note 3: Unavailability may also be expressed mathematically as the ratio of the total time a functional unit is not capable of being used during a given interval to the length of the interval, e.g., if the unit is not capable of being used for 68 hours a week, the unavailability is 68/168.

unavailability /'ʌnə,veilə'biliti/
danh từ tính chất không sẵn sàng để dùng; tính chất không dùng được sự không có giá trị (của vé xe...)
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