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 = a1aISOL10 (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 .