Multivariate Classes in Reliability Theory.

Four classes of lifetimes which have been useful in describing situations where systems are assumed to have independent univariate component lifetimes are the increasing failure rate IFR class the increasing failure rate average IFRA class the new better than used NBU class and the new better than used in expectation NBUE class. These classes are reviewed and also multivariate analogs of the IFR and IFRA cases are discussed. New multivariate definitions of NBU and NBUE are introduced.