### Definition(s)

#### Unreliability **3.1.9**

*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 *t*f 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 *t*f 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*