On optimality of extremal schemes in progressive type II censoring

In a progressively type II censored life-testing experiment intact units may be removed from the experiment after every failure. If the initial number of units in the experiment and the total number of failures are fixed, the experimenter may choose between different censoring schemes. By specifying an optimality criterion, one may improve the outcome of the experiment by choosing the respective optimal scheme. We establish a simple property of a general optimality criterion that yields optimality of certain extremal schemes. Applications to some criteria that measure the total time of the experiment and its variability illustrate the approach. The results are based on stochastic orderings of generalized order statistics.