| In this thesis, it is shown that the characterizations for DRHR class in Shaked and Shanthikumar (1994) are in fact sufficient conditions by a counter example. Sufficient conditions for DRHR and IMIT classes are given. We also investigate the inactivity time of k-out-of-n systems based on DRHR and IMIT classes. In addition, by using inactivity time to characterize the second-price closed-seal auction rent, we investigate how the variation of the auction rents reals that of populations. |