A Note on the Comparisons among Coherent Systems

 Using the concept of system signature introduced by Samaniego (1985), Kochar et al. (1999) compared the lifetimes of the systems in which the lifetimes of the components are independent and identically distributed (i.i.d.) random variables. Their results are extended to the systems with exchangeable components by Navarro et al. (2005). This paper gives some alternative proofs to obtain their results. Particularly in view of the hazard rate ordering, we compare two systems with different structures and components, which extends Theorem 8 in Navarro et al. (2005). We also compare two systems with different structures and components in view of the likelihood ratio ordering. Some illustrative examples are mentioned.