Published online by Cambridge University Press: 14 July 2016
A new and simpler proof of Morrison's result that within exponential mixtures only IFR gamma mixing produces linearly increasing mean residual life functions is given. A parallel and new characterization of the DFR gamma laws follows as a consequence. The method of proof used suggests a general result on the infinite divisibility of the mixing distributions in exponential mixtures.
Research supported by NRC Grant number A-8218 of the National Research Council of Canada, while the author was visiting Laurentian University.