Unreliability

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F(t)

〈measure〉 probability for an item to fail to perform a required function under given conditions over a

given time interval [0, t]

Note 1 to entry: F(t) is also the probability that the time of the first failure tf is lower than t: F(t)= P(t _ t) f . This is

in relationship with the occurrence of the first failure.

Note 2 to entry: F(t) is the cdf (cumulative distribution function) of the time to the first failure tf of the item. It

ranges from 0 to 1 when t goes from 0 to infinity.

Note 3 to entry: The unreliability is the complementary of the reliability: F(t)=1−R(t)

Note 4 to entry: When dealing with safety, F(t) is generally small compared to 1 and this property is used to

develop approximated formulae (see Clause 7).

Note 5 to entry: Unreliability is better to communicate than MTTF.

Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards

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