Manufacturers and consumers prefer reliable products‎, ‎or systems in general‎, ‎since they need to assure that systems work satisfactory for given mission times‎. ‎The redundancy allocation to original system components is a common technique to improve reliabilities‎. ‎But‎, ‎allocation of redundants is not an easy task and must be considered properly with respect to environmental working conditions and possible restrictions such as cost‎, ‎volume and weight‎. ‎Therefore‎, ‎the problem of finding optimal allocations is important and studied extensively in literature‎. ‎The existing studies usually assume restrictive conditions such as stochastically independent component and spare lifetimes‎. ‎This article deals with this problem under a general setting in which component and spare lifetimes can be dependent and heterogeneous‎. ‎Two common policies, ‎called active and standby, ‎are studied in details‎. ‎Stochastic orders are implemented for comparing various allocation policies‎. ‎Findings of this article are derived under general conditions and hold for arbitrary dependency structures among lifetimes‎. ‎Illustrative examples are also given‎.

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