  Several preconditioned AOR methods have been proposed to solve system of
linear equations $Ax=b$, where $A \in \mathbb{R}^{n \times n}$ is a unit
Z-matrix. The aim of this paper is to give a comparison result for a class of
preconditioners $P$, where $P\in \mathbb{R}^{n\times n}$ is nonsingular,
nonnegative and has unit diagonal entries. Numerical results for corresponding
preconditioned GMRES methods are given to illustrate the theoretical results.
