〈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