ISO 281:2007 introduced a life modification factor, a_ISO, based on a systems approach to life calculation, in addition to the life modification factor, a₁.These factors are applied in the modified rating life equation, L_nm = a₁a_ISOL₁₀. The modification factor for reliability, a₁, for a range of reliability values, is given in ISO 281:2007 as well as the method for evaluating the modification factor for systems approach, a_ISO. L₁₀ is the basic rating life.

ISO/TR 1281-2:2008 gives supplementary background information regarding the derivation of a₁ and a_ISO.