Virtual age of non-repairable objects

Two definitions of virtual age are considered. The first one is based on the fact that a system’s deterioration depends on an environment. In a more severe environment deterioration is more intensive and, therefore, the corresponding virtual age is larger than the calendar age. The second approach defines virtual age at the moment of switching from one regime to another. It is shown that both definitions coincide only for the linear scale transformation in the lifetime distribution functions. r 2008 Elsevier Ltd. All rights reserved.