The number of failures of an item per unit measure of life (cycles, time, etc., as appropriate). For any particular item the failure rate will be based on the results of life tests and one or more of the following will be quoted.

• • The observed failure rate is the ratio of the total number of failures in a single population to the sum of the times during which each item has been subjected to stress conditions. It is associated with particular and stated time intervals and stress conditions. The criteria for what constitutes a failure should be stated.

• • The assessed failure rate is determined as a limiting value of the confidence interval with a stated probability level based on the same data as the observed failure rate of nominally identical items. The following conditions apply: the source of the data should be stated; results may be combined only when all conditions are similar; it should be stated whether one- or two-sided intervals are being used; the upper limiting value is usually used for failure-rate statements and the assumed underlying distribution should be stated.

• • The extrapolated failure rate is an extension of the observed or assessed failure rate by defined extrapolation or interpolation for durations or stress conditions different from those applying to the conditions of the assessed rate.

• • The failure rate of an item varies during its lifetime (see diagram). In the early failure period the failure rate decreases rapidly. To avoid an early failure in use a manufacturer will operate and test an item in a process, known as burn-in, that stabilizes its characteristics. The failure rate increases rapidly in the wear-out failure period due to deterioration processes. The useful life of an item is the period from a stated time during which, under stated conditions, an item has an acceptable failure rate. For items showing a failure rate pattern as in the diagram, the useful life corresponds to the constant failure-rate period, when failures occur at an approximately uniform rate. See also mean time between failures; mean time to failure.

Change of failure rate with time