It is shown that that the mean sample spacings E(Xi+1:n-Xi:n) is nonincreasing in the sample size if distribution is DRHR and E(Xi+1:n-Xi:n)-E(Xi+2:n+1-Xi+1:n+1) is nonnegative if distribution is IFR.

• 证明了当X具有DRHR年龄性质时；E(Xk+1:n-Xk:n)关于样本容量n递减.；而当X具有IFR年龄性质时；E(Xk+1:n-Xk:n)-E(Xk+2:n+1-Xk+1:n+1)非负