ISO 281:2007 introduced a life modification factor, aISO, based on a systems approach to life calculation, in
addition to the life modification factor for reliability, a1.These factors are applied in the modified rating life
equation
Lnm = a1 aISO L10 (1)
For a range of reliability values, a1 is given in ISO 281:2007 as well as the method for evaluating the
modification factor for systems approach, aISO. L10 is the basic rating life.
This part of ISO/TR 1281 gives supplementary background information regarding the derivation of a1 and aISO.
NOTE The derivation of aISO is primarily based on theory presented in Reference [5], which also deals with the fairly
complicated theoretical background of the contamination factor, eC, and other factors considered when calculating aISO .