Closure property of the NBUC class under formation of parallel systems

Reference [7] proves that the family of NBUC distributions is closed under the formation of parallel systems with i.i.d components. This paper proves, through a simple argument based on stochastic orders, that this property also holds for s-independent but not identically distributed components. The proof presented here, with minor modifications, also provides both the proof of the closure under formation of series systems of NBU(2) distributions class, and a more intuitively appealing method to prove the well-known closure under formation of s-coherent systems of the NBU distributions class.