New results on comparisons of parallel systems with heterogeneous gamma components

This paper discusses ordering properties of lifetimes of parallel systems with two independent heterogeneous gamma components in terms of dispersive and star orders. It is proved, among others, that the pp-larger order between the two scale vectors implies the dispersive order and the star order between lifetimes of two parallel systems. Another sufficient condition is also provided for the star order between the lifetimes of two parallel systems. The results established here extend some of the known results in the literature.