This standard specifies procedures to verify the assumption of a constant failure rate or constant failure intensity, as defined in IEC 60050(191), and to identify patterns in the failure rate or intensity. These procedures are applicable whenever it is necessary to verify such assumptions. This may be due to a requirement or for the purpose of assessing any variation with time of the failure rate or failure intensity. The objectives of the methods specified in this standard are as follows: – to test whether the times to failure of non-repaired items are exponentially distributed, i.e. the failure rate is constant; – to test whether the times between failures of repaired item(s) have any time trend, i.e. the failure intensity does not exhibit an increasing or decreasing trend; – to construct graphs that allow the patterns in the failure rate or failure intensity to be displayed, with a view to verifying whether they can be assumed constant, to estimate their values or to identify the nature of any departure from constancy.