Group sequential methods based on ranked set samples

Ranked set sampling (RSS) schemes were originally proposed to increase efficiency in estimation. On the other hand, group sequential methods provide substantial savings in sample and enable us to make decisions as early as possible. In this manuscript, we intend to combine the benefits of the two methodologies. We propose group sequential tests for one and two population means under RSS. We compare the power, average sample sizes and type I errors of the proposed tests to those of group sequential tests based on simple random sampling schemes. We illustrate the utility of the method by using data from HIV trial.