On Stochastic Comparisons of Order Statistics from Heterogeneous Exponential Samples

We show that the $k$th order statistic from a heterogeneous sample of $n\geq k$ exponential random variables is larger than that from a homogeneous exponential sample in the sense of star ordering, as conjectured by Xu and Balakrishnan (2012). As a consequence, we establish hazard rate ordering for order statistics between heterogeneous and homogeneous exponential samples, resolving an open problem of P\v{a}lt\v{a}nea (2008). Extensions to general spacings are also presented.