Block replacement policy, under which items are replaced at constant intervals of time and at failure, is wasteful because sometimes almost new items are also replaced. In order to eliminate this kind of waste, the following replacement policy is suggested.
(1) Items are exchanged for new items at time kT (k ··· 1, 2, …).
(2) If items fail in [(k – 1) T, kT – v), they are replaced by new items, and if in [kT – v, kT), they are replaced by used items.
The results of this ‘extended block replacement policy with used items' are studied for the Erlang lifetime distribution.